OTTR / Otter Tail Corporation - Arquivamentos da SEC, Relatório Anual, Declaração de Procuração

Otter Tail Corporation
US ˙ NasdaqGS ˙ US6896481032

Estatísticas Básicas
LEI 549300HHVBQRQUVKKD91
CIK 1466593
SEC Filings
All companies that sell securities in the United States must register with the Securities and Exchange Commission (SEC) and file reports on a regular basis. These reports include company annual reports (10K, 10Q), news updates (8K), investor presentations (found in 8Ks), insider trades (form 4), ownership reports (13D, and 13G), and reports related to the specific securities sold, such as registration statements and prospectus. This page shows recent SEC filings related to Otter Tail Corporation
SEC Filings (Chronological Order)
Esta página fornece uma lista completa e cronológica de Arquivamentos da SEC, excluindo os arquivamentos de propriedade que fornecemos em outro lugar.
May 29, 2026 SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission File Number) 27-0383995 (I.R.S. Employer Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 5

May 29, 2026 EX-1.01

Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2025

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2025 Introduction This report for the year ended December 31, 2025, is made by Otter Tail Corporation (the "Company") in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the "Rule"). The Rule was adopted by the Securities and Exchange Commission ("SEC") to implement reporting and discl

May 29, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 28, 2026 OTTER TAIL CORPORATI

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 28, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 15, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 15, 2026 OTTER TAIL CORPORATI

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 15, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 6, 2026 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended March 31, 2026 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact name of registra

May 6, 2026 EX-10.1

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 20[ ] EXECUTIVE PERFORMANCE AWARD AGREEMENT (CASH-SETTLED)

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 20[ ] EXECUTIVE PERFORMANCE AWARD AGREEMENT (CASH-SETTLED) This Performance Award Agreement is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and you, as an employee of the Corporation, effective as of the date of grant (the “Grant Date”) set forth in the attached Performance Award Certificate.

May 5, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 4, 2026 OTTER TAIL CORPORATIO

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 4, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 5, 2026 EX-99.1

2025 EPS by Segment

Press Release May 4, 2026 Otter Tail Corporation Announces First Quarter Earnings and Affirms 2026 EPS Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended March 31, 2026.

April 15, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 13, 2026 OTTER TAIL CORPORA

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 13, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

April 14, 2026 EX-99.1

Otter Tail Corporation Announces President and CFO Transitions

Otter Tail Corporation Announces President and CFO Transitions Date: April 14, 2026 FERGUS FALLS, Minn.

April 14, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 13, 2026 OTTER TAIL CORPORA

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 13, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 23, 2026 EX-10.1

Otter Tail Power Company $100,000,000 5.33% Series 2026A Senior Unsecured Notes due March 19, 2036 $70,000,000 6.04% Series 2026B Senior Unsecured Notes due June 4, 2056 Note Purchase Agreement Dated as of March 19, 2026 TABLE OF CONTENTS

Otter Tail Power Company $100,000,000 5.33% Series 2026A Senior Unsecured Notes due March 19, 2036 $70,000,000 6.04% Series 2026B Senior Unsecured Notes due June 4, 2056 Note Purchase Agreement Dated as of March 19, 2026 TABLE OF CONTENTS Section Page SECTION 1. AUTHORIZATION OF NOTES 1 SECTION 2. SALE AND PURCHASE OF NOTES 1 SECTION 3. EXECUTION; CLOSINGS 1 SECTION 4. CONDITIONS TO CLOSINGS 2 Sec

March 23, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 19, 2026 OTTER TAIL CORPORA

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 19, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 2, 2026 DEF 14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☑ Defin

March 2, 2026 ARS

Annual Report 2025 S H A R E H O L D E R S E RV I C ES 215 South Cascade Street P.O. Box 496 Fergus Falls, MN 56538 Email: [email protected] www.ottertail.com | Nasdaq: OTTR O TTER TA IL CO RPO RATIO N 2025 A N N U A L REPO RT Welcome to our 202

a2025annualreportottrno Annual Report 2025 S H A R E H O L D E R S E RV I C ES 215 South Cascade Street P.

March 2, 2026 DEFA14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☐ Defin

February 23, 2026 10-K/A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K/A Amendment No. 1

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K/A Amendment No. 1 (Mark One) ☒ Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2025 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 18, 2026 EX-10.18

OTTER TAIL CORPORATION 1999 EMPLOYEE STOCK PURCHASE PLAN (amended April 10, 2006, April 16, 2012, September 22, 2016 and October 1, 2025) ARTICLE I. INTRODUCTION

OTTER TAIL CORPORATION 1999 EMPLOYEE STOCK PURCHASE PLAN (amended April 10, 2006, April 16, 2012, September 22, 2016 and October 1, 2025) ARTICLE I.

February 18, 2026 EX-19

OTTER TAIL CORPORATION INSIDER TRADING AND PRE-CLEARANCE POLICY

Page | 1 OTTER TAIL CORPORATION INSIDER TRADING AND PRE-CLEARANCE POLICY I. PURPOSE OF POLICY The Insider Trading and Pre-Clearance Policy (the “Policy”) is intended to avoid situations where purchases or sales of stock or other securities are consummated by persons who are in possession of material non-public information (“MNPI”). The Policy provides guidelines with respect to transactions in sec

February 18, 2026 EX-10.24

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 EXECUTIVE PERFORMANCE AWARD AGREEMENT (CASH-SETTLED)

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 EXECUTIVE PERFORMANCE AWARD AGREEMENT (CASH-SETTLED) This Performance Award Agreement is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and you, as an employee of the Corporation, effective as of the date of grant (the “Grant Date”) set forth in the attached Performance Award Certificate.

February 18, 2026 PRE 14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☑ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☐ Defin

February 18, 2026 EX-21

OTTER TAIL CORPORATION Subsidiaries of the Registrant February 18, 2026

Exhibit 21 OTTER TAIL CORPORATION Subsidiaries of the Registrant February 18, 2026 Company State of Organization BTD Manufacturing, Inc. Minnesota Northern Pipe Products, Inc. North Dakota Otter Tail Assurance Limited Cayman Islands Otter Tail Power Company Minnesota T.O. Plastics, Inc. Minnesota Varistar Corporation Minnesota Vinyltech Corporation Arizona

February 18, 2026 EX-24

POWER OF ATTORNEY

Exhibit 24 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 18, 2026 EX-10.29

SUMMARY OF NON-EMPLOYEE DIRECTOR COMPENSATION (APRIL 2025)

SUMMARY OF NON-EMPLOYEE DIRECTOR COMPENSATION (APRIL 2025) Compensation: Non-employee directors shall receive the following remuneration: Director’s Retainer (except Chairman) $ 80,000 Chairman of the Board Retainer $ 148,000 Committee Chair Retainer (per committee, except the Audit Committee Chair) $ 15,000 Audit Committee Chair Retainer $ 21,000 Committee Non-Chair Retainer (per committee) $ 10,

February 18, 2026 EX-10.26

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 RESTRICTED STOCK UNIT AWARD AGREEMENT FOR EXECUTIVE OFFICERS

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 RESTRICTED STOCK UNIT AWARD AGREEMENT FOR EXECUTIVE OFFICERS This Restricted Stock Unit Award Agreement for Executive Officers is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and the person named in the attached Restricted Stock Unit Award Certificate who is an Executive Officer of the Corporation effective as of the date of grant (the “Grant Date”) set forth in the attached Restricted Stock Unit Award Certificate.

February 18, 2026 EX-10.23

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 EXECUTIVE PERFORMANCE AWARD AGREEMENT (STOCK SETTLED)

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 EXECUTIVE PERFORMANCE AWARD AGREEMENT (STOCK SETTLED) This Performance Award Agreement is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and you, as an employee of the Corporation, effective as of the date of grant (the “Grant Date”) set forth in the attached Performance Award Certificate.

February 18, 2026 10-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2025 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 18, 2026 EX-10.21

Otter Tail Corporation Executive Annual Incentive Plan February 10, 2026

Otter Tail Corporation Executive Annual Incentive Plan February 10, 2026 BACKGROUND The Compensation and Human Capital Management Committee (hereinafter the “Compensation Committee”) of Otter Tail Corporation (the “Corporation”) has established the Executive Annual Incentive Plan (the “Plan”) to reward top Corporate executives (“Participants”) who are directly involved in ensuring that the Corporation reaches its performance objectives.

February 18, 2026 EX-10.27

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 RESTRICTED STOCK UNIT AWARD AGREEMENT FOR EXECUTIVE OFFICERS (CASH-SETTLED)

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2026 RESTRICTED STOCK UNIT AWARD AGREEMENT FOR EXECUTIVE OFFICERS (CASH-SETTLED) This Restricted Stock Unit Award Agreement for Executive Officers is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and the person named in the attached Restricted Stock Unit Award Certificate who is an Executive Officer of the Corporation effective as of the date of grant (the “Grant Date”) set forth in the attached Restricted Stock Unit Award Certificate.

February 17, 2026 EX-99.1

2025 EPS by Segment

Press Release February 16, 2026 Otter Tail Corporation Announces Annual Earnings and Initiates 2026 Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter and year ended December 31, 2025.

February 17, 2026 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 16, 2026 OTTER TAIL CORP

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 16, 2026 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

December 18, 2025 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 16, 2025 OTTER TAIL CORP

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 16, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

December 18, 2025 EX-99.1

December 18, 2025

December 18, 2025 Otter Tail Corporation appoints Chris Clark and Steve Rasche to Board of Directors Steve Fritze to retire at April Annual Meeting of Shareholders Fergus Falls, MN – Otter Tail Corporation has appointed Mr.

November 5, 2025 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended September 30, 2025 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact name of regi

November 4, 2025 EX-99.1

2024 EPS by Segment

Press Release November 3, 2025 Otter Tail Corporation Announces Third Quarter Earnings, Increases Annual Earnings Guidance and Uplifts Long-Term Financial Targets FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended September 30, 2025.

November 4, 2025 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 3, 2025 OTTER TAIL CORPO

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 3, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

August 6, 2025 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended June 30, 2025 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact name of registran

August 5, 2025 EX-99.1

2024 EPS by Segment

Press Release August 4, 2025 Otter Tail Corporation Announces Second Quarter Earnings and Increases Annual Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended June 30, 2025.

August 5, 2025 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 4, 2025 OTTER TAIL CORPORA

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 4, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

June 6, 2025 8-K

Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): June 5, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

June 5, 2025 8-K

Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): June 4, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 30, 2025 EX-1.01

Conflict Minerals Report for the reporting period January 1, 202

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2024 Introduction This report for the year ended December 31, 2024, is made by Otter Tail Corporation (the "Company") in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the "Rule"). The Rule was adopted by the Securities and Exchange Commission ("SEC") to implement reporting and discl

May 30, 2025 SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission File Number) 27-0383995 (I.R.S. Employer Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 5

May 7, 2025 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended March 31, 2025 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exac

May 6, 2025 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 5, 2025 OTTER TAIL CORPORATIO

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 5, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 6, 2025 EX-99.1

2024 EPS by Segment

Press Release May 5, 2025 Otter Tail Corporation Announces First Quarter Earnings and Affirms Annual Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended March 31, 2025.

April 17, 2025 8-K

Submission of Matters to a Vote of Security Holders

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 14, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 31, 2025 8-K

Entry into a Material Definitive Agreement, Financial Statements and Exhibits, Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 27, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 31, 2025 EX-4.1

and the Purchasers named therein (incorporated by reference to Exhibit 4.1 to the Form 8-K filed by Otter Tail Corporation on

Otter Tail Power Company $50,000,000 5.49% Series 2025A Senior Unsecured Notes due March 27, 2035 $50,000,000 5.98% Series 2025B Senior Unsecured Notes due June 5, 2055 Note Purchase Agreement Dated as of March 27, 2025 TABLE OF CONTENTS Section Page SECTION 1. AUTHORIZATION OF NOTES 1 SECTION 2. SALE AND PURCHASE OF NOTES 1 SECTION 3. EXECUTION; CLOSINGS 1 SECTION 4. CONDITIONS TO CLOSINGS 2 Sect

March 3, 2025 ARS

Annual Report 2024 S H A R E H O L D E R S E RV I C ES 215 S. Cascade St., P.O. Box 496 Fergus Falls, MN 56538-0496 Phone: 800-664-1259 or 218-739-8479 Email: [email protected] www.ottertail.com | Nasdaq: OTTR Vision We build top-performing comp

Annual Report 2024 S H A R E H O L D E R S E RV I C ES 215 S. Cascade St., P.O. Box 496 Fergus Falls, MN 56538-0496 Phone: 800-664-1259 or 218-739-8479 Email: [email protected] www.ottertail.com | Nasdaq: OTTR Vision We build top-performing companies in a diversified organization with an electric utility as our foundation. Mission We deliver value by building strong electric utility and manuf

March 3, 2025 DEF 14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☑ Defin

March 3, 2025 DEFA14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☐ Defin

February 19, 2025 EX-19

and Pre-Clearance

Page | 1 OTTER TAIL CORPORATION INSIDER TRADING AND PRE-CLEARANCE POLICY I. PURPOSE OF POLICY The Insider Trading and Pre-Clearance Policy (the “Policy”) is intended to avoid situations where purchases or sales of stock or other securities are consummated by persons who are in possession of material non-public information (“MNPI”). The Policy provides guidelines with respect to transactions in sec

February 19, 2025 EX-10.20

Executive Annual Incentive Plan

Otter Tail Corporation Executive Annual Incentive Plan February 11, 2025 BACKGROUND The Compensation and Human Capital Management Committee (hereinafter the “Compensation Committee”) of Otter Tail Corporation (the “Corporation”) has established the Executive Annual Incentive Plan (the “Plan”) to reward top Corporate executives (“Participants”) who are directly involved in ensuring that the Corporation reaches its performance objectives.

February 19, 2025 EX-21

Subsidiaries of Registrant

Exhibit 21 OTTER TAIL CORPORATION Subsidiaries of the Registrant February 19, 2025 Company State of Organization BTD Manufacturing, Inc. Minnesota Northern Pipe Products, Inc. North Dakota Otter Tail Assurance Limited Cayman Islands Otter Tail Power Company Minnesota T.O. Plastics, Inc. Minnesota Varistar Corporation Minnesota Vinyltech Corporation Arizona

February 19, 2025 EX-10.21

Form of Executive Performance Share Award Agreement (Executives)*

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2025 EXECUTIVE PERFORMANCE AWARD AGREEMENT This Performance Award Agreement is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and you, as an employee of the Corporation, effective as of the date of grant (the “Grant Date”) set forth in the attached Performance Award Certificate.

February 19, 2025 EX-10.24

Summary of Non-Employee Director Compensation (202

SUMMARY OF NON-EMPLOYEE DIRECTOR COMPENSATION (APRIL 2024) Compensation: Non-employee directors shall receive the following remuneration: Director’s Retainer (except Chairman) $ 80,000 Chairman of the Board Retainer $ 148,000 Compensation and Corporate Governance Committee Chair Retainer $ 15,000 Audit Committee Chair Retainer $ 21,000 Restricted Stock Award Value (except Chairman) $ 120,000 Restricted Stock Award Value (Chairman of the Board) $ 162,000 Restricted Stock Award Value (additional per committee) $ 10,500 Directors may elect to receive their retainer (other than expense reimbursements) in the form of cash, stock or a combination.

February 19, 2025 10-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2024 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 19, 2025 EX-24

Power of Attorney

Exhibit 24 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 19, 2025 EX-10.22

Form of Restricted Stock Unit Award Agreement (Executives)*

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2025 RESTRICTED STOCK UNIT AWARD AGREEMENT FOR EXECUTIVE OFFICERS This Restricted Stock Unit Award Agreement for Executive Officers is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and the person named in the attached Restricted Stock Unit Award Certificate who is an Executive Officer of the Corporation effective as of the date of grant (the “Grant Date”) set forth in the attached Restricted Stock Unit Award Certificate.

February 18, 2025 EX-99.1

2024 EPS by Segment

Press Release February 17, 2025 Otter Tail Corporation Announces Record Annual Earnings and Increases Long-Term Financial Targets FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter and year ended December 31, 2024.

February 18, 2025 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 17, 2025 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

December 31, 2024 8-K

Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 30, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

December 16, 2024 EX-10.2

, 2024, by and between Otter Tail Power Company, as Borrower, and the banks named therein, with U.S. Bank National Association, as Administrative Agent

FIFTH AMENDED AND RESTATED CREDIT AGREEMENT DATED AS OF DECEMBER 11, 2024 BETWEEN OTTER TAIL POWER COMPANY, as Borrower, the LENDERS, U.

December 16, 2024 8-K

Entry into a Material Definitive Agreement, Financial Statements and Exhibits, Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 11, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

December 16, 2024 EX-10.1

, 2024, by and between Otter Tail Corporation, as Borrower, and the banks named therein, with U.S. Bank National Association, as Administrative Agent

SIXTH AMENDED AND RESTATED CREDIT AGREEMENT DATED AS OF DECEMBER 11, 2024 BETWEEN OTTER TAIL CORPORATION, as Borrower, the LENDERS, U.

November 7, 2024 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended September 30, 2024 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (

November 5, 2024 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 4, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

November 5, 2024 EX-99.1

2024 EPS Guidance

EX-99.1 2 q32024earningsreleaseex991.htm EX-99.1 Press Release November 4, 2024 Otter Tail Corporation Announces Third Quarter Earnings, Increases Consolidated 2024 Annual Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended September 30, 2024. SUMMARY •Midpoint of 2024 earnings guidance increased $0.15 to $7.07 p

October 3, 2024 SC 13D/A

OTTR / Otter Tail Corporation / CASCADE INVESTMENT, L.L.C. - SC 13D/A Activist Investment

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 SCHEDULE 13D Under the Securities Exchange Act of 1934 (Amendment No. 12)* Otter Tail Corporation (Name of Issuer) Common Shares, par value $5.00 per share (Title of Class of Securities) 689648103 (CUSIP Number) Jeff Shim, Esq. 2365 Carillon Point Kirkland, WA 98033 (425) 889-7900 (Name, Address and Telephone Number of Persons

August 7, 2024 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended June 30, 2024 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact

August 6, 2024 EX-99.1

2024 EPS Guidance

Press Release August 5, 2024 Otter Tail Corporation Announces Record Second Quarter Earnings, Maintains Electric Segment Guidance and Increases Consolidated 2024 Annual Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended June 30, 2024.

August 6, 2024 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 5, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

June 5, 2024 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 30, 2024 OTTER TAIL CORPORATI

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 30, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 24, 2024 SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission File Number) 27-0383995 (I.R.S. Employer Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 5

May 24, 2024 EX-1.01

Conflict Minerals Report for the reporting period January 1, 202

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2023 Introduction This report for the year ended December 31, 2023 is made by Otter Tail Corporation (the Company) in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the Rule). The Rule was adopted by the Securities and Exchange Commission (SEC) to implement reporting and disclosure r

May 8, 2024 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended March 31, 2024 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exac

May 7, 2024 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 6, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 7, 2024 EX-99.1

2024 EPS Guidance

Press Release May 6, 2024 Otter Tail Corporation Announces First Quarter Earnings and Increases 2024 Annual Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter ended March 31, 2024.

May 3, 2024 S-3ASR

As filed with the Securities and Exchange Commission on May 3, 2024

As filed with the Securities and Exchange Commission on May 3, 2024 Registration No 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.

May 3, 2024 EX-FILING FEES

ing Fee Table

Exhibit 107 Calculation of Filing Fee Tables FORM S-3ASR (Form Type) OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Table 1: Newly Registered and Carry Forward Securities Security Type Security Class Title Fee Calculation Rule or Carry Forward Rule Amount Registered Proposed Maximum Offering Price Per Unit Maximum Aggregate Offering Price Fee Rate Amount of Registrat

May 3, 2024 S-3ASR

As filed with the Securities and Exchange Commission on May 3, 2024

As filed with the Securities and Exchange Commission on May 3, 2024 Registration No 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.

May 3, 2024 EX-25.1

—Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 on Form T-1 of U.S. Bank National Association, as Trustee under the Indenture (for Unsecured Debt Securities).

Exhibit 25-A UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM T-1 STATEMENT OF ELIGIBILITY UNDER THE TRUST INDENTURE ACT OF 1939 OF A CORPORATION DESIGNATED TO ACT AS TRUSTEE Check Application to Determine Eligibility of a Trustee Pursuant to Section 305(b)(2) ☐ U.S. BANK TRUST COMPANY, NATIONAL ASSOCIATION (Exact name of Trustee as specified in its charter) 91-1821036

May 3, 2024 EX-24.1

Power of Attorney

Exhibit 24.1 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Charles S. MacFarlane, Todd R. Wahlund and Jennifer O. Smestad, and each or any one of them, his or her true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution, for him or her and in his or her name, place and ste

May 3, 2024 EX-FILING FEES

Filing Fee Table

Exhibit 107 Calculation of Filing Fee Tables FORM S-3ASR (Form Type) OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Table 1: Newly Registered and Carry Forward Securities Security Type Security Class Title Fee Calculation Rule or Carry Forward Rule Amount Registered(1)(2) Proposed Maximum Offering Price Per Unit(3) Maximum Aggregate Offering Price Fee Rate Amount of Registration Fee Newly Registered Securities Fees to be Paid Equity Common Shares, par value $5 per share 457(c) 1,500,000 $85.

May 3, 2024 EX-24.1

—Power of Attorney.

Exhibit 24-A POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Charles S.

April 10, 2024 8-K

Submission of Matters to a Vote of Security Holders

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 8, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissio

April 5, 2024 8-K

Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 2, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissio

April 2, 2024 EX-4.1

ent dated as of March 28, 2024 between Otter Tail Power and the Purchasers nam

Otter Tail Power Company $60,000,000 5.48% Series 2024A Senior Unsecured Notes due April 1, 2034 $60,000,000 5.77% Series 2024B Senior Unsecured Notes due April 1, 2054 Note Purchase Agreement Dated as of March 28, 2024 TABLE OF CONTENTS Section Page SECTION 1. AUTHORIZATION OF NOTES. 1 SECTION 2. SALE AND PURCHASE OF NOTES. 1 SECTION 3. CLOSING. 1 SECTION 4. CONDITIONS TO CLOSING. 2 Section 4.1.

April 2, 2024 8-K

Entry into a Material Definitive Agreement, Financial Statements and Exhibits, Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 28, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

February 28, 2024 DEFA14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☐ Defin

February 28, 2024 DEF 14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☑ Defin

February 28, 2024 ARS

Annual Report 2023 We deliver value by building strong electric utility and manufacturing platforms. FOR OUR SHAREHOLDERS we deliver above-average returns through commercial and operational excellence and growing our businesses. FOR OUR CUSTOMERS we

Annual Report 2023 We deliver value by building strong electric utility and manufacturing platforms.

February 14, 2024 EX-10.28

Otter Tail Corporation Executive Severance Plan (

OTTER TAIL CORPORATION EXECUTIVE SEVERANCE PLAN Effective February 6, 2015 Schedule A amended on January 1, 2018, and January 1, 2024 OTTER TAIL CORPORATION EXECUTIVE SEVERANCE PLAN TABLE OF CONTENTS SECTION 1 - INTRODUCTION 1 SECTION 2 - DEFINITIONS 1 SECTION 3 - ELIGIBILITY FOR SEVERANCE 2 SECTION 4 - AMOUNT OF SEVERANCE PAY AND OTHER BENEFITS 3 SECTION 5 - WHEN SEVERANCE PAY WILL BE PAID 3 SECTION 6 - MISCELLANEOUS PROVISIONS 3 SECTION 7 - ADMINISTRATION 5 2 OTTER TAIL CORPORATION EXECUTIVE SEVERANCE PLAN SECTION 1 - INTRODUCTION This Otter Tail Corporation Executive Severance Plan (the “Plan”) is effective for the benefit of designated Employees of Otter Tail Corporation (the “Corporation”) and its affiliates.

February 14, 2024 EX-10.33

Consulting Agreement, Kevin G. Moug, dated January 8, 2024

CONSULTING AGREEMENT THIS CONSULTING AGREEMENT executed this 8th day of January 2024, by and between Otter Tail Corporation (“Company”) a Minnesota corporation, and Kevin G.

February 14, 2024 EX-10.20

Summary of Non-Employee Director Compensation (202

SUMMARY OF NON-EMPLOYEE DIRECTOR COMPENSATION (APRIL 2023) Compensation: Non-employee directors shall receive the following remuneration: Director’s Retainer (except Chairman) $ 80,000 Chairman of the Board Retainer $ 148,000 Compensation and Corporate Governance Committee Chair Retainer $ 15,000 Audit Committee Chair Retainer $ 21,000 Restricted Stock Award Value (except Chairman) $ 110,000 Restricted Stock Award Value (Chairman of the Board) $ 136,500 Restricted Stock Award Value (additional per committee) $ 10,500 Directors may elect to receive their retainer (other than expense reimbursements) in the form of cash, stock or a combination.

February 14, 2024 10-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2023 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 14, 2024 EX-21

Subsidiaries of Registrant.

Exhibit 21 OTTER TAIL CORPORATION Subsidiaries of the Registrant February 14, 2024 Company State of Organization BTD Manufacturing, Inc. Minnesota Northern Pipe Products, Inc. North Dakota Otter Tail Assurance Limited Cayman Islands Otter Tail Power Company Minnesota T.O. Plastics, Inc. Minnesota Varistar Corporation Minnesota Vinyltech Corporation Arizona

February 14, 2024 EX-10.15

Nonqualified Retirement Plan (20

OTTER TAIL CORPORATION NONQUALIFIED RETIREMENT PLAN 2021 RESTATEMENT i 2261523v11 4836-4002-8627\5 TABLE OF CONTENTS ARTICLE 1.

February 14, 2024 EX-10.25

Change in Control Severance Agreement, Todd Wahlund, dated January 1, 2024.*

CHANGE IN CONTROL SEVERANCE AGREEMENT This Change in Control Severance Agreement (the “Agreement”) is made as of the 1st day of January, 2024, between Otter Tail Corporation, a Minnesota corporation, with its principal offices at 215 South Cascade Street, P.

February 14, 2024 EX-10.32

nit Award Agreement (Executives)

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2023 RESTRICTED STOCK UNIT AWARD AGREEMENT FOR EXECUTIVE OFFICERS This Restricted Stock Unit Award Agreement for Executive Officers is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and the person named in the attached Restricted Stock Unit Award Certificate who is an Executive Officer of the Corporation effective as of the date of grant (the “Grant Date”) set forth in the attached Restricted Stock Unit Award Certificate.

February 14, 2024 EX-10.31

2023 Executive Performance Share Award Agreement

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2023 EXECUTIVE PERFORMANCE AWARD AGREEMENT This Performance Award Agreement is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and you, as an employee of the Corporation, effective as of the date of grant (the “Grant Date”) set forth in the attached Performance Award Certificate.

February 14, 2024 EX-24

Power of Attorney.

Exhibit 24 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 14, 2024 EX-97

Incentive Compensation Recovery Policy

Exhibit 97 OTTER TAIL CORPORATION INCENTIVE COMPENSATION RECOVERY POLICY 1.Introduction. The Board of Directors of Otter Tail Corporation (the “Company”) believes that it is in the best interests of the Company and its shareholders to create and maintain a culture that emphasizes integrity and accountability and that reinforces the Company's compensation philosophy. The Board has therefore adopted

February 14, 2024 EX-10.26

Change in Control Severance Agreement, Jennifer Smestad, dated January 1, 2018.*

CHANGE IN CONTROL SEVERANCE AGREEMENT This Change in Control Severance Agreement (the “Agreement”) is made as of the 1st day of January, 2018, between Otter Tail Corporation, a Minnesota corporation, with its principal offices at 215 South Cascade Street, P.

February 13, 2024 EX-99.1

2023 EPS by Segment

Press Release February 12, 2024 Otter Tail Corporation Announces Record Annual Earnings, Increases Quarterly Dividend, and Announces 2024 Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter and year ended December 31, 2023.

February 13, 2024 SC 13G/A

OTTR / Otter Tail Corporation / VANGUARD GROUP INC - SCHEDULE 13G/A Passive Investment

SC 13G/A 1 tv0106-ottertailcorp.htm SCHEDULE 13G/A SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 13)* Name of issuer: Otter Tail Corp Title of Class of Securities: Common Stock CUSIP Number: 689648103 Date of Event Which Requires Filing of this Statement: December 29, 2023 Check the appropriate box to designate the

February 13, 2024 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 12, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

January 29, 2024 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): January 26, 2024 OTTER TAIL CORPO

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): January 26, 2024 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

January 22, 2024 SC 13G/A

OTTR / Otter Tail Corporation / BlackRock Inc. Passive Investment

us6896481032012224.txt SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No: 10) OTTER TAIL CORP - (Name of Issuer) Common Stock - (Title of Class of Securities) 689648103 - (CUSIP Number) December 31, 2023 - (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to w

December 21, 2023 8-K

Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 15, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

November 3, 2023 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended September 30, 2023 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (

October 31, 2023 EX-99.1

2023 EPS Guidance July 31, 2023

Press Release October 30, 2023 Otter Tail Corporation Announces Record Quarterly Earnings, Increases 2023 Earnings Guidance, Board of Directors Declares Quarterly Dividend of $0.

October 31, 2023 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): October 30, 2023 OTTER TAIL CORPO

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): October 30, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

October 4, 2023 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): October 2, 2023 OTTER TAIL CORPOR

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): October 2, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commiss

October 4, 2023 EX-99.1

Otter Tail Corporation CFO and SVP Moug to Retire: Wahlund Named Successor

Otter Tail Corporation CFO and SVP Moug to Retire: Wahlund Named Successor Date: October 4, 2023 FERGUS FALLS, Minn.

August 2, 2023 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended June 30, 2023 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact

August 1, 2023 EX-99.1

# # #

Press Release July 31, 2023 Otter Tail Corporation Announces Second Quarter Earnings, Increases 2023 Earnings Guidance, Board of Directors Declares Quarterly Dividend of $0.

August 1, 2023 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): July 31, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissio

June 20, 2023 8-K/A

Submission of Matters to a Vote of Security Holders

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K/A (Amendment No. 1) CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 16, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organizatio

May 25, 2023 EX-1.01

Conflict Minerals Report for the reporting period January 1, 202

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2022 Introduction This report for the year ended December 31, 2022 is made by Otter Tail Corporation (the Company) in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the Rule). The Rule was adopted by the Securities and Exchange Commission (SEC) to implement reporting and disclosure r

May 25, 2023 SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission File Number) 27-0383995 (I.R.S. Employer Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 5

May 3, 2023 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended March 31, 2023 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exac

May 2, 2023 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 1, 2023 OTTER TAIL CORPORATIO

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 1, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 2, 2023 EX-99.1

# # #

Press Release May 1, 2023 Otter Tail Corporation Announces First Quarter Earnings, Increases 2023 Earnings Guidance, Board of Directors Declares Quarterly Dividend of $0.

April 19, 2023 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 16, 2023 OTTER TAIL CORPORA

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 16, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

April 19, 2023 EX-10.2

Form of 2023 Restricted Stock Award Agreement for Directors

OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN 2023 RESTRICTED STOCK AWARD AGREEMENT FOR DIRECTORS This Restricted Stock Award Agreement is between Otter Tail Corporation, a Minnesota corporation (the “Corporation”), and the person named in the attached Restricted Stock Award Certificate for Directors who is a Non-employee Director (“Director”) of the Corporation effective as of the date of grant (the “Grant Date”) set forth in the attached Restricted Stock Award Certificate for Directors.

April 19, 2023 EX-10.3

Executive Annual Incentive Plan

Exhibit 10.3 Otter Tail Corporation Executive Annual Incentive Plan April 16, 2023 BACKGROUND The Compensation and Human Capital Management Committee (hereinafter the “Compensation Committee”) of Otter Tail Corporation (the “Corporation”) has established the Executive Annual Incentive Plan (the “Plan”) to reward top Corporate executives (“Participants”) who are directly involved in ensuring that t

April 17, 2023 EX-FILING FEES

Filing Fee Table

EXHIBIT 107 Calculation of Filing Fee Tables FORM S-8 (Form Type) OTTER TAIL CORPORATION (Exact Name of Registrant as Specified in its Charter) Table 1: Newly Registered and Carry Forward Securities Security Type Security Class Title(1) Fee Calculation Rule Amount Registered Proposed Maximum Offering Price Per Unit Maximum Aggregate Offering Price Fee Rate Amount of Registration Fee Newly Registered Securities Equity Common Shares, par value $5 per share Rule 457(h) 979,891(2) $70.

April 17, 2023 EX-4.1

2023 Stock Incentive Plan

Exhibit 4.1 OTTER TAIL CORPORATION 2023 STOCK INCENTIVE PLAN Section 1. Purpose. The purpose of the Plan is to promote the interests of the Company and its shareholders by aiding the Company in attracting and retaining employees, officers, consultants, independent contractors, advisors and non‑employee Directors capable of assuring the future success of the Company, to offer such persons incentive

April 17, 2023 S-8

As filed with the Securities and Exchange Commission on April 17, 2023

S-8 1 forms-8xregstmtx2023stocki.htm S-8 As filed with the Securities and Exchange Commission on April 17, 2023 Registration No 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM S-8 REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporat

March 31, 2023 8-K

Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 31, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 8, 2023 SC 13G/A

OTTR / Otter Tail Corporation / BlackRock Inc. Passive Investment

us6896481032030723.txt SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No: 9) OTTER TAIL CORP - (Name of Issuer) Common Stock - (Title of Class of Securities) 689648103 - (CUSIP Number) February 28, 2023 - (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to wh

March 7, 2023 DEFA14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

DEFA14A 1 a2023proxy-noticeofinterne.htm DEFA14A UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commissio

March 7, 2023 DEFR14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. 1 )

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. 1 ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) ☑ Def

March 1, 2023 DEF 14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14

February 15, 2023 EX-10.28

Summary of Non-Employee Director Compensation (20

SUMMARY OF NON-EMPLOYEE DIRECTOR COMPENSATION (April 2022) Compensation: Non-employee directors shall receive the following remuneration: Director’s Retainer (except Chairman) $ 76,000 Chairman of the Board Retainer $ 144,000 Compensation and Corporate Governance Committee Chair Retainer $ 15,000 Audit Committee Chair Retainer $ 21,000 Restricted Stock Award Value (except Chairman) $ 95,000 Restricted Stock Award Value (Chairman of the Board) $ 121,500 Restricted Stock Award Value (additional per Committee) $ 10,500 Directors may elect to receive their retainer (other than expense reimbursements) in the form of cash, stock or a combination.

February 15, 2023 EX-24

Power of Attorney.

Exhibit 24 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 15, 2023 10-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2022 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 15, 2023 EX-10.36

Form of Change in Control Severance Agreement (2023)*

Exhibit 10.36 CHANGE IN CONTROL SEVERANCE AGREEMENT This Change in Control Severance Agreement (the “Agreement”) is made as of the day of [month], [year], between Otter Tail Corporation, a Minnesota corporation, with its principal offices at 215 South Cascade Street, P.O. Box 496, Fergus Falls, Minnesota 56538‑0496 (the “Corporation “) and [name] (“You”), residing at [address], and supersedes any

February 15, 2023 EX-21

Subsidiaries of Registrant.

Exhibit 21 OTTER TAIL CORPORATION Subsidiaries of the Registrant February 15, 2023 Company State of Organization BTD Manufacturing, Inc. Minnesota Northern Pipe Products, Inc. North Dakota Otter Tail Assurance Limited Cayman Islands Otter Tail Power Company Minnesota T.O. Plastics, Inc. Minnesota Varistar Corporation Minnesota Vinyltech Corporation Arizona

February 14, 2023 EX-99.1

# # #

Press Release February 13, 2023 Otter Tail Corporation Announces Fourth Quarter Earnings, Record Annual Earnings, Increases Quarterly Dividend and Announces 2023 Earnings Guidance FERGUS FALLS, Minnesota - Otter Tail Corporation (Nasdaq: OTTR) today announced financial results for the quarter and year ended December 31, 2022.

February 14, 2023 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 13, 2023 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

February 9, 2023 SC 13G/A

OTTR / Otter Tail Corporation / VANGUARD GROUP INC - SCHEDULE 13G/A Passive Investment

SC 13G/A 1 tv0097-ottertailcorp.htm SCHEDULE 13G/A SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 12)* Name of issuer: Otter Tail Corp. Title of Class of Securities: Common Stock CUSIP Number: 689648103 Date of Event Which Requires Filing of this Statement: December 30, 2022 Check the appropriate box to designate the

January 31, 2023 SC 13G/A

OTTR / Otter Tail Corporation / BlackRock Inc. Passive Investment

SC 13G/A 1 us6896481032013123.txt us6896481032013123.txt SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No: 8) OTTER TAIL CORP - (Name of Issuer) Common Stock - (Title of Class of Securities) 689648103 - (CUSIP Number) December 31, 2022 - (Date of Event Which Requires Filing of this Statement) Check the appropriate box to

December 22, 2022 EX-99.1

Otter Tail Corporation Appoints Ms. Jeanne Crain and Ms. Mary Ludford to Board of Directors

Otter Tail Corporation Appoints Ms. Jeanne Crain and Ms. Mary Ludford to Board of Directors Mr. James Stake to retire at April Annual Meeting of Shareholders Company Release - 12/22/2022 FERGUS FALLS, Minnesota?(BUSINESS WIRE)?Otter Tail Corporation (?the Company?) has appointed Ms. Jeanne Crain of Minneapolis, Minnesota, and Ms. Mary Ludford of Chicago, Illinois, to the company?s Board of Directo

December 22, 2022 8-K

Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers, Financial Statements and Exhibits

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 21, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

November 2, 2022 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ? Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended September 30, 2022 or ? Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (

November 1, 2022 EX-99.1

# # #

Press Release October 31, 2022 Otter Tail Corporation Announces Record Third Quarter Earnings and Updated Guidance, Midpoint of Reduced 2022 Earnings Per Share Guidance Represents a 55% Increase Over 2021 Earnings Per Share, Board of Directors Declares Quarterly Dividend of $0.

November 1, 2022 EX-10.2

Fourth Amended and Restated Credit Agreement, dated as of October 31, 2022, by and between Otter Tail Power Company, as Borrower, and the banks named therein, with U.S. Bank Nation Association, as Administration Agent.

FOURTH AMENDED AND RESTATED CREDIT AGREEMENT by and between, OTTER TAIL POWER COMPANY, as Borrower, and U.

November 1, 2022 EX-10.1

Fifth Amended and Restated Credit Agreement, dated as of October 31, 2022, by and between Otter Tail Corporation, as Borrower, and the banks named therein, with U.S. Bank National Association, as Administrative Agent.

FIFTH AMENDED AND RESTATED CREDIT AGREEMENT by and between, OTTER TAIL CORPORATION, as Borrower, and U.

November 1, 2022 8-K

Entry into a Material Definitive Agreement, Financial Statements and Exhibits, Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): October 31, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

October 19, 2022 8-K

Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): October 14, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

September 23, 2022 8-K

Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers, Financial Statements and Exhibits

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): September 21, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Comm

August 3, 2022 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended June 30, 2022 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact

August 2, 2022 EX-99.1

# # #

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 Investor contact: Tyler Akerman, Manager of Investor Relations, (800) 664-1259 For release: August 1, 2022 Financial Media Otter Tail Corporation Announces Record Second Quarter Earnings, Increases 2022 Earnings Guidance, Board of Directors Declares Quarterly Dividend of $0.4125 per Share

August 2, 2022 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 1, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

May 27, 2022 SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission File Number) 27-0383995 (I.R.S. Employer Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 5

May 27, 2022 EX-1.01

Conflict Minerals Report for the reporting period January 1, 202

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2021 Introduction This report for the year ended December 31, 2021 is made by Otter Tail Corporation (the Company) in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the Rule). The Rule was adopted by the Securities and Exchange Commission (SEC) to implement reporting and disclosure r

May 23, 2022 8-K

Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 20, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 4, 2022 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ? Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended March 31, 2022 or ? Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exac

May 3, 2022 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 2, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 3, 2022 EX-99.1

# # #

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 Investor contact: Tyler Akerman, Manager of Investor Relations, (800) 664-1259 For release: May 2, 2022 Financial Media Otter Tail Corporation Announces Record First Quarter Earnings, Increases 2022 Earnings Guidance, Board of Directors Declares Quarterly Dividend of $0.4125 per Share FERG

April 14, 2022 8-K

Submission of Matters to a Vote of Security Holders

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 11, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 2, 2022 DEF 14A

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. )

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ? Filed by a Party other than the Registrant ? Check the appropriate box: ? Preliminary Proxy Statement ? Confidential, for Use of the Commission Only (as permitted by Rule 14

February 16, 2022 EX-21

Subsidiaries of Registrant.

Exhibit 21 OTTER TAIL CORPORATION Subsidiaries of the Registrant February 16, 2022 Company State of Organization BTD Manufacturing, Inc. Minnesota Northern Pipe Products, Inc. North Dakota Otter Tail Assurance Limited Cayman Islands Otter Tail Power Company Minnesota T.O. Plastics, Inc. Minnesota Varistar Corporation Minnesota Vinyltech Corporation Arizona

February 16, 2022 EX-24

Power of Attorney.

Exhibit 24 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 16, 2022 10-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ? Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2021 or ? Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 15, 2022 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 14, 2022 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

February 15, 2022 EX-99.1

# # #

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 Investor contact: Tyler Akerman, Manager of Investor Relations, (800) 664-1259 For release: February 14, 2022 Financial Media Otter Tail Corporation Announces Fourth Quarter Earnings, Increases Quarterly Dividend 5.8% and Announces 2022 Earnings Guidance of $3.78 to $4.08 per share FERGUS

February 9, 2022 SC 13G/A

OTTR / Otter Tail Corporation / VANGUARD GROUP INC - SCHEDULE 13G/A Passive Investment

SC 13G/A 1 tv0086-ottertailcorp.htm SCHEDULE 13G/A SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 11)* Name of issuer: Otter Tail Corp. Title of Class of Securities: Common Stock CUSIP Number: 689648103 Date of Event Which Requires Filing of this Statement: December 31, 2021 Check the appropriate box to designate the

February 3, 2022 SC 13G/A

OTTR / Otter Tail Corporation / BlackRock Inc. Passive Investment

us6896481032020322.txt SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No: 7) OTTER TAIL CORP - (Name of Issuer) Common Stock - (Title of Class of Securities) 689648103 - (CUSIP Number) December 31, 2021 - (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to wh

December 15, 2021 8-K

Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers, Financial Statements and Exhibits

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 14, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

December 15, 2021 EX-99.1

Pictured above: O’Keefe

Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Tyler Akerman, Manager of Investor Relations, (218) 998-7110 or (800) 664-1259 For release: December 15, 2021 Otter Tail Corporation appoints Dr.

December 1, 2021 8-K

Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 29, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

November 3, 2021 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended September 30, 2021 or ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (

November 2, 2021 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 1, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commis

November 2, 2021 EX-99.1

# # #

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Tyler Akerman, Manager of Investor Relations, (218) 998-7110 or (800) 664-1259 For release: November 1, 2021 Financial Media Otter Tail Corporation Announces Third Quarter Earnings and Increases 2021 Earnings Per Share Guidance Driven by Continued Strong

October 4, 2021 EX-10.2

Amended And Restated Credit Agreement, dated as of September 30, 2021, by and between Otter Tail

Exhibit 10.2 THIRD AMENDED AND RESTATED CREDIT AGREEMENT by and between, OTTER TAIL POWER COMPANY, as Borrower, and U.S. BANK NATIONAL ASSOCIATION, as Administrative Agent, and BANK OF AMERICA, N.A., as Syndication Agent, and KEYBANC CAPITAL MARKETS INC. and WELLS FARGO BANK, NATIONAL ASSOCIATION, as Co-Documentation Agents Dated as of September 30, 2021 U.S. BANK NATIONAL ASSOCIATION, BOFA SECURI

October 4, 2021 EX-10.1

Fourth Amended And Restated Credit Agreement, dated as of September 30, 2021, by and between Otter Tail Corporation,

Exhibit 10.1 FOURTH AMENDED AND RESTATED CREDIT AGREEMENT by and between, OTTER TAIL CORPORATION, as Borrower, and U.S. BANK NATIONAL ASSOCIATION, as Administrative Agent, and KEYBANC CAPITAL MARKETS INC., as Syndication Agent, and BANK OF AMERICA, N.A. and WELLS FARGO BANK, NATIONAL ASSOCIATION, as Co-Documentation Agents Dated as of September 30, 2021 U.S. BANK NATIONAL ASSOCIATION, KEYBANC CAPI

October 4, 2021 8-K

Entry into a Material Definitive Agreement, Financial Statements and Exhibits, Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): September 30, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Comm

September 2, 2021 EX-99.1

Cost and reliability drive Otter Tail Power Company’s 15-year resource plan

Exhibit 99.1 News release Contact: Stephanie Hoff [email protected] 218-739-8535 (office) 218-205-6179 (after hours) Cost and reliability drive Otter Tail Power Company?s 15-year resource plan Fergus Falls, Minnesota, September 1, 2021: Today?Otter Tail Power Company?submitted its Integrated Resource Plan (IRP) to regulatory commissions in each of the three states it serves: Minnesota, North Dakota,

September 2, 2021 8-K

Financial Statements and Exhibits, Other Events

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): September 1, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

August 6, 2021 10-Q

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ? Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly period ended June 30, 2021 or ? Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact

August 3, 2021 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 2, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

August 3, 2021 EX-99.1

# # #

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Tyler Akerman, Manager of Investor Relations, (218) 998-7110 or (800) 664-1259 For release: August 2, 2021 Financial Media Otter Tail Corporation Announces Second Quarter Earnings and Increases 2021 Earnings Per Share Guidance Board of Directors Declares

June 14, 2021 EX-4.1

between Otter Tail Power Company and the Purchasers named therein (incorporated by reference to Exhibit 4.1 to the Form 8-K filed by Otter Tail Corporation on

OTTER TAIL POWER COMPANY $40,000,000 2.74% Series 2021A Senior Unsecured Notes due November 29, 2031 $100,000,000 3.69% Series 2021B Senior Unsecured Notes due November 29, 2051 $90,000,000 3.77% Series 2022A Senior Unsecured Notes due May 20, 2052 NOTE PURCHASE AGREEMENT Dated as of June 10, 2021 TABLE OF CONTENTS Section Page SECTION 1. AUTHORIZATION OF NOTES. 1 SECTION 2. SALE AND PURCHASE OF N

June 14, 2021 8-K

Entry into a Material Definitive Agreement, Financial Statements and Exhibits, Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): June 10, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissio

May 28, 2021 EX-1.01

Conflict Minerals Report for the reporting period January 1, 20

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2020 Introduction This report for the year ended December 31, 2020 is made by Otter Tail Corporation (the Company) in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the Rule). The Rule was adopted by the Securities and Exchange Commission (SEC) to implement reporting and disclosure r

May 28, 2021 SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter)

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission File Number) 27-0383995 (I.R.S. Employer Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 5

May 10, 2021 SC 13G/A

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 10)*

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 10)* Name of issuer: Otter Tail Corp. Title of Class of Securities: Common Stock CUSIP Number: 689648103 Date of Event Which Requires Filing of this Statement: April 30, 2021 Check the appropriate box to designate the rule pursuant to which this Schedule is filed: ??Rule

May 7, 2021 10-Q

Quarterly Report - 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ? Quarterly Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the quarterly ended March 31, 2021 or ? Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact name

May 4, 2021 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 3, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commission

May 4, 2021 EX-99.1

# # #

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: May 3, 2021 Financial Media Otter Tail Corporation Announces First Quarter Earnings and Increases 2021 Earnings Per Share Guidance Board of Directors Declares Quar

May 3, 2021 S-3ASR

- S-3

As filed with the Securities and Exchange Commission on May 3, 2021 Registration No 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.

May 3, 2021 S-3ASR

- S-3ASR

As filed with the Securities and Exchange Commission on May 3, 2021 Registration No 333- UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.

May 3, 2021 EX-5.A

—Consent of Dorsey & Whitney LLP (included in Exhibit 5.1).

Exhibit 5-A [Letterhead of Dorsey & Whitney LLP] Otter Tail Corporation 215 South Cascade Street Fergus Falls, Minnesota 56538 Re: Registration Statement on Form S-3 Ladies and Gentlemen: We have acted as counsel to Otter Tail Corporation, a Minnesota corporation (the ?Company?), in connection with a Registration Statement on Form S-3 (the ?Registration Statement?) filed by the Company with the Se

May 3, 2021 EX-25.A

Trust Indenture Act of 1939 on Form T-1 of U.S. Bank National Association, as Trustee under the Indenture (for Unsecured Debt Securities).

Exhibit 25-A UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM T-1 STATEMENT OF ELIGIBILITY UNDER THE TRUST INDENTURE ACT OF 1939 OF A CORPORATION DESIGNATED TO ACT AS TRUSTEE Check Application to Determine Eligibility of a Trustee Pursuant to Section 305(b)(2) ? U.S. BANK NATIONAL ASSOCIATION (Exact name of Trustee as specified in its charter) 31-0841368 (I.R.S. Employe

May 3, 2021 EX-23.A

—Consent of Deloitte & Touche LLP.

Exhibit 23-A CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM We consent to the incorporation by reference in this Registration Statement on Form S-3 of our report dated February 19, 2021 relating to the financial statements of Otter Tail Corporation and subsidiaries, and the effectiveness of Otter Tail Corporation and subsidiaries? internal control over financial reporting, appearing in the Annual Report on Form 10-K of Otter Tail Corporation and subsidiaries for the year ended December 31, 2020.

May 3, 2021 EX-24.A

—Power of Attorney.

Exhibit 24-A POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Charles S.

May 3, 2021 EX-24.1

Power of Attorney

Exhibit 24.1 POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Charles S. MacFarlane, Kevin G. Moug and Jennifer O. Smestad, and each or any one of them, his or her true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution, for him or her and in his or her name, place and stead

April 14, 2021 EX-3.1

Articles of Incorporation

Exhibit 3.1 THIRD RESTATED ARTICLES OF INCORPORATION OF OTTER TAIL CORPORATION (restated as of April 12, 2021) ARTICLE I. The name of the corporation shall be Otter Tail Corporation. ARTICLE II. The purposes of the corporation shall be as follows: (a)To generate, produce, buy or in any manner acquire, and to sell, dispose of, and distribute electricity for light, heat and power and other purposes,

April 14, 2021 EX-3.2

Restated Bylaws

Exhibit 3.2 RESTATED BYLAWS OF OTTER TAIL CORPORATION (restated as of April 12, 2021) ARTICLE I. OFFICES, CORPORATE SEAL Section 1.01. Offices. The registered office of the corporation in Minnesota and the principal executive office shall be at 215 South Cascade Street, Fergus Falls, Minnesota 56537. The corporation may have such other offices, within or without the State of Minnesota, as the dire

April 14, 2021 8-K

Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year, Financial Statements and Exhibits, Submission of Matters to a Vote of Security Holders

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 12, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commissi

March 3, 2021 DEF 14A

- DEF 14A

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ? Filed by a Party other than the Registrant ? Check the appropriate box: ? Preliminary Proxy Statement ? Confidential, for Use of the Commission Only (as permitted by Rule 14

February 19, 2021 EX-21.-A

—Subsidiaries of Registrant.

Exhibit 21-A OTTER TAIL CORPORATION Subsidiaries of the Registrant February 19, 2021 Company State of Organization BTD Manufacturing, Inc. Minnesota Northern Pipe Products, Inc. North Dakota Otter Tail Assurance Limited Cayman Islands Otter Tail Power Company Minnesota T.O. Plastics, Inc. Minnesota Varistar Corporation Minnesota Vinyltech Corporation Arizona

February 19, 2021 EX-24.-A

—Power of Attorney.

Exhibit 24-A POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 19, 2021 EX-23.-A

—Consent of Deloitte & Touche LLP.

Exhibit 23-A CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM We consent to the incorporation by reference in Registration Statement Nos.

February 19, 2021 10-K

Annual Report - 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ? Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2020 or ? Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Commission File Number 0-53713 OTTER TAIL CORPORATION (Exact nam

February 17, 2021 PRE 14A

- PRE 14A

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ? Filed by a Party other than the Registrant ? Check the appropriate box: ? Preliminary Proxy Statement ? Confidential, for Use of the Commission Only (as permitted by Rule 14

February 16, 2021 EX-99.1

2020 EPS

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: February 15, 2021 Financial Media Otter Tail Corporation Reports a 7.8 Percent Increase in 2020 Diluted Earnings per Share to $2.34, Increases Quarterly Dividend 5

February 16, 2021 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 15, 2021 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota (State or other jurisdiction of incorporation or organization) 0-53713 (Commi

February 10, 2021 SC 13G/A

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 9)*

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 9)* Name of issuer: Otter Tail Corp. Title of Class of Securities: Common Stock CUSIP Number: 689648103 Date of Event Which Requires Filing of this Statement: December 31, 2020 Check the appropriate box to designate the rule pursuant to which this Schedule is filed: ☒ Ru

January 29, 2021 SC 13G/A

SC 13G/A

us6896481032012921.txt SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No: 6) OTTER TAIL CORP - (Name of Issuer) Common Stock - (Title of Class of Securities) 689648103 - (CUSIP Number) December 31, 2020 - (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to wh

November 6, 2020 10-Q

Quarterly Report - FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended September 30, 2020 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission file numbe

November 3, 2020 EX-99.1

Line Limit

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: November 2, 2020 Financial Media Otter Tail Corporation Announces 40% Increase in Third Quarter Earnings Per Share Raises and Narrows its 2020 Earnings Per Share G

November 3, 2020 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition - FORM 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 2, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission

November 2, 2020 EX-99.1

###

Exhibit 99.1 Otter Tail Power Company requests rate review in Minnesota Fergus Falls, MN, November 2, 2020: Today Otter Tail Power Company filed a request to increase its rates with the Minnesota Public Utilities Commission (PUC). The filing starts an approximately 18-month process, often referred to as a rate case, during which the PUC reviews the costs the company incurs to provide customers wit

November 2, 2020 8-K

Regulation FD Disclosure, Financial Statements and Exhibits - FORM 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): November 2, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission

August 20, 2020 8-K

Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant - FORM 8-K

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 20, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission F

August 7, 2020 10-Q

Quarterly Report - FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended June 30, 2020 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission file number 0-5

August 4, 2020 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): August 3, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission Fi

August 4, 2020 EX-99.1

Line Limit

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: August 3, 2020 Financial Media Otter Tail Corporation Announces Second Quarter Earnings Increases 2020 Earnings Per Share Guidance Range to $2.10-$2.30 Board of Di

May 29, 2020 EX-1.01

Conflict Minerals Report for the reporting period January 1, 2019 to December 31, 2019 as required by Items 1.01 and 1.02 of this Form SD.

Exhibit 1.01 Conflict Minerals Report Otter Tail Corporation For the Year Ended December 31, 2019 Introduction This report for the year ended December 31, 2019 is made by Otter Tail Corporation (the Company) in compliance with Rule 13p-1 under the Securities Exchange Act of 1934 (the Rule). The Rule was adopted by the Securities and Exchange Commission (SEC) to implement reporting and disclosure r

May 29, 2020 SD

- FORM SD

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM SD SPECIALIZED DISCLOSURE REPORT OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction (Commission (I.R.S. Employer of incorporation) File Number) Identification No.) 215 South Cascade Street, P.O. Box 496, Fergus Falls, MN 56538-0496 (Addre

May 8, 2020 10-Q

Quarterly Report - FORM 10-Q

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2020 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission file number 0-

May 6, 2020 8-K

Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers, Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): May 5, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission File

May 6, 2020 EX-99.1

Line Limit

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: May 5, 2020 Financial Media Otter Tail Corporation Announces First Quarter Earnings Electric and Plastics Segments' Guidance Remain Largely Unchanged; Revised 2020

April 22, 2020 8-K

Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers, Financial Statements and Exhibits, Submission of Matters to a Vote of Security Holders

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): April 20, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission Fi

April 22, 2020 EX-10.1

Otter Tail Corporation Executive Restoration Plus Plan, 2020 Restatement

Exhibit 10.1 OTTER TAIL CORPORATION EXECUTIVE RESTORATION PLUS PLAN As Amended and Restated on April 20, 2020 TABLE OF CONTENTS SECTION 1. Purpose and Administration 1 1.1. Name of Plan 1.2. Effective Date 1.3. Purpose 1.4. Administration SECTION 2. Definitions 2 2.1. Account 2.2. Affiliate 2.3. Bonus 2.4. Change in Control 2.5. Code 2.6. Committee 2.7. Company 2.8. Compensation 2.9. Deferral Elec

March 20, 2020 8-K

Financial Statements and Exhibits, Other Events

-12-31 UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 20, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commis

March 20, 2020 EX-99.1

For release: March 20, 2020

Exhibit 99.1 Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: March 20, 2020 Otter Tail Corporation moves Annual Meeting of Shareholders to virtual format Fergus Falls, MN – Due to the emerging public health threat of the global coronavir

March 20, 2020 DEFA14A

OTTR / Otter Tail Corp. DEFA14A - - DEFA14A

DEFA14A UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 (Amendment No. ) Filed by the Registrant ☑ Filed by a Party other than the Registrant ☐ Check the appropriate box: ☐ Preliminary Proxy Statement ☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

March 20, 2020 EX-3.1

—Restated Bylaws.

Exhibit 3.1 RESTATED BYLAWS OF OTTER TAIL CORPORATION Article I. OFFICES, CORPORATE SEAL Section 1.01. Offices. The registered office of the corporation in Minnesota and the principal executive office shall be at 215 South Cascade Street, Fergus Falls, Minnesota 56537. The corporation may have such other offices, within or without the State of Minnesota, as the directors shall, from time to time,

March 17, 2020 8-K

Financial Statements and Exhibits

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): March 17, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission Fi

March 17, 2020 EX-99.1

For release: March 17, 2020

Exhibit 99.1 Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: March 17, 2020 Otter Tail Corporation activates plan to address COVID-19 Fergus Falls, MN – Otter Tail Corporation (the Company) has activated a company-wide plan to address po

March 9, 2020 DEF 14A

OTTR / Otter Tail Corp. DEF 14A - - DEF 14A

DEF 14A Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.

February 26, 2020 8-K

Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 25, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission

February 24, 2020 SC 13D/A

OTTR / Otter Tail Corp. / Cascade Investment, L.L.C. - SC 13D/A Activist Investment

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13D Under the Securities Exchange Act of 1934 (Amendment No. 11)* Otter Tail Corporation (Name of Issuer) Common Shares, par value $5.00 per share (Title of Class of Securities) 689648103 (CUSIP Number) Jacki Badal, Esq. 2365 Carillon Point Kirkland, WA 98033 (425) 889-7900 (Name, Address and Telephone Number of Perso

February 20, 2020 EX-23.A

—Consent of Deloitte & Touche LLP.

Exhibit 23-A CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM We consent to the incorporation by reference in Registration Statement Nos.

February 20, 2020 10-K

Annual Report - FORM 10-K

Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the fiscal year ended December 31, 2019 ☐ Transition Report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the transition period from to Commission File Number 0-53713 OT

February 20, 2020 EX-21.A

—Subsidiaries of Registrant.

Exhibit 21-A OTTER TAIL CORPORATION Subsidiaries of the Registrant February 20, 2020 Company State of Organization Otter Tail Power Company Minnesota Otter Tail Assurance Limited Cayman Islands Varistar Corporation Minnesota Northern Pipe Products, Inc. North Dakota Vinyltech Corporation Arizona T.O. Plastics, Inc. Minnesota BTD Manufacturing, Inc. Minnesota

February 20, 2020 EX-24.A

—Power of Attorney.

Exhibit 24-A POWER OF ATTORNEY KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints CHARLES S.

February 20, 2020 EX-4.I

Description of Securities

Exhibit 4-I DESCRIPTION OF THE REGISTRANT’S SECURITIES REGISTERED PURSUANT TO SECTION 12 OF THE SECURITIES EXCHANGE ACT OF 1934 As of December 31, 2019, the sole class of securities of Otter Tail Corporation (the “Company”) registered under Section 12 of the Securities Exchange Act of 1934, as amended, was the Company’s common shares.

February 18, 2020 EX-99.1

Line Limit

Exhibit 99.1 NEWS RELEASE Media contact: Stephanie Hoff, Director of Corporate Communications, (218) 739-8535 or (218) 205-6179 Investor contact: Loren Hanson, Manager of Investor Relations, (218) 739-8481 or (800) 664-1259 For release: February 17, 2020 Financial Media Otter Tail Corporation Reports a 5.3 Percent Increase in 2019 Diluted Earnings per Share to $2.17, Increases Quarterly Dividend 5

February 18, 2020 8-K

Financial Statements and Exhibits, Results of Operations and Financial Condition

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): February 17, 2020 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission

February 12, 2020 SC 13G/A

OTTR / Otter Tail Corp. / VANGUARD GROUP INC - SCHEDULE 13G/A Passive Investment

SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G Under the Securities Exchange Act of 1934 (Amendment No.: 8)* Name of issuer: Otter Tail Corp Title of Class of Securities: Common Stock CUSIP Number: 689648103 Date of Event Which Requires Filing of this Statement: December 31, 2019 Check the appropriate box to designate the rule pursuant to which this Schedule is filed: ☒ Rul

February 5, 2020 SC 13G/A

OTTR / Otter Tail Corp. / BlackRock Inc. Passive Investment

us6896481032020520.txt SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No: 5) OTTER TAIL CORP - (Name of Issuer) Common Stock - (Title of Class of Securities) 689648103 - (CUSIP Number) December 31, 2019 - (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to wh

December 27, 2019 8-K

Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers, Financial Statements and Exhibits

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of report (Date of earliest event reported): December 26, 2019 OTTER TAIL CORPORATION (Exact name of registrant as specified in its charter) Minnesota 0-53713 27-0383995 (State or other jurisdiction of incorporation) (Commission

December 27, 2019 EX-10.1

Executive Survivor and Supplemental Retirement Plan (2020 Restatement).*

Exhibit 10.1 OTTER TAIL CORPORATION EXECUTIVE SURVIVOR AND SUPPLEMENTAL RETIREMENT PLAN (2020 Restatement) OTTER TAIL CORPORATION EXECUTIVE SURVIVOR AND SUPPLEMENTAL RETIREMENT PLAN (2020 Restatement) TABLE OF CONTENTS Page SECTION 1. INTRODUCTION 1 1.1. Plan Established 1.2. Amendment and Restatement 1.3. Unfunded Obligation 1.4. Compliance With Section 409A of the Code SECTION 2. DEFINITIONS 2 2

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