OTTERTAIL CORPORATION AMENDMENT NO. 5 DATED AS OF DECEMBER 12, 2011 to NOTE PURCHASE AGREEMENT Dated as of February 23, 2007 RE: $50,000,000 SENIOR NOTE due November 30, 2017 AMENDMENT NO. 5 TO NOTEPURCHASE AGREEMENT

Exhibit 4.1

OTTER TAIL CORPORATION

AMENDMENT NO. 5

DATED AS OF DECEMBER 12, 2011

to

NOTE PURCHASE AGREEMENT

Dated as of February 23, 2007

RE: $50,000,000 SENIOR NOTE

due November 30, 2017

AMENDMENT NO. 5 TO NOTE PURCHASE AGREEMENT

THIS AMENDMENT NO. 5 dated as of December 12, 2011 (the or this “Amendment”) to the Note Purchase Agreement dated as of February 23, 2007 is between Otter Tail Corporation, a Minnesota corporation (the “Company”), and Cascade Investment, L.L.C. (“Cascade”).

RECITALS:

A. The Company and Cascade have heretofore entered into the Note Purchase Agreement, dated as of February 23, 2007, as amended by a letter agreement dated December 14, 2007, an Amendment No. 2 dated as of June 30, 2009 (“Amendment No. 2”), an Amendment No. 3 dated as of June 23, 2010, and an Amendment No. 4 dated as of July 29, 2010 (as so amended, the “Note Purchase Agreement”). The Company has heretofore issued the $50,000,000 8.89% Senior Note due November 30, 2017 (the “Note”) pursuant to the Note Purchase Agreement.

B. The Company and Cascade now desire to amend the Note Purchase Agreement in the respects, but only in the respects, hereinafter set forth.

C Capitalized terms used herein shall have the respective meanings ascribed thereto in the Note Purchase Agreement (as amended hereby) unless herein defined or the context shall otherwise require.

NOW, THEREFORE, in consideration of good and valuable consideration the receipt and sufficiency of which is hereby acknowledged, the Company and Cascade do hereby agree as follows, which agreement shall become effective as of the Effective Time (as defined below) upon the full and complete satisfaction of each of the conditions precedent set forth in Section 3.1 hereof:

ARTICLE I

AMENDMENT

Section 1.1. Effective as of the Effective Time, the definition of “Consolidated Net Income” set forth in Annex A to the Note Purchase Agreement shall be amended as follows:

(a) by deleting clause (e) thereof in its entirety and replacing with clause (e) as set forth below:

“(e) non-operating gains (including, without limitation, extraordinary or nonrecurring gains, gains from the sale of fixed assets related to the discontinuance of operations or the sale of a subsidiary or business unit that constitutes a division of the Company), and similar non-operating losses during the applicable period; provided, however, that gains and losses resulting from the sale of fixed assets during such period that is not related to the discontinuance of operations or the sale of a subsidiary or business unit that constitutes a division of the Company shall be included in Consolidated Net Income;”

(b) by amending clause (j) thereof to read as follows:

“(j) any net income or gain during such period from (i) any change in accounting principles in accordance with GAAP, (ii) any prior period adjustments resulting from any change in accounting principles in accordance with GAAP; and in the case of each of clauses (i) and (ii), any similar aggregate net loss;”

ARTICLE II

REPRESENTATIONS AND WARRANTIES OF THE COMPANY

Section 2.1. To induce Cascade to execute and deliver this Amendment (which representations shall survive the execution and delivery of this Amendment), the Company represents and warrants to Cascade that:

(a) (i) this Amendment has been duly authorized by all requisite corporate action on the part of the Company, and (ii) this Amendment has been executed and delivered by the Company and constitutes the legal, valid and binding agreement of the Company enforceable against it in accordance with its terms, except as enforcement may be limited by bankruptcy, insolvency, reorganization, moratorium or similar laws or equitable principles relating to or limiting creditors’ rights generally;

(b) each of the Note Purchase Agreement, as amended by this Amendment, and the Note, constitutes the legal, valid and binding obligation, contract and agreement of the Company enforceable against it in accordance with its terms, except as enforcement may be limited by bankruptcy, insolvency, reorganization, moratorium or similar laws or equitable principles relating to or limiting creditors’ rights generally;

(c) the execution and delivery by the Company of this Amendment and the performance by the Company of its obligations under this Amendment will not (A) violate the Articles of Incorporation of the Company, as amended, or Bylaws of the Company, as amended, (B) violate Section 673 of the Minnesota Business Corporation Act (the “MBCA”) or Minnesota Statutes Section 216B.48, or (C) violate, result in the breach or modification of, conflict with, constitute a default or result in an acceleration of any obligation under, result in the imposition of any encumbrance pursuant to, or affect the validity or effectiveness of, any contract, permit, order or other law applicable to the Company, except (as to clause (C) only) for any violation, breach, modification, conflict, default, acceleration, encumbrance or effect which would not have a material adverse effect on the Company and its Subsidiaries taken as a whole. No approval or consent, filings, notifications, waivers or exemptions on the part of any (A) Minnesota, North Dakota or South Dakota or (B) New York or federal, governmental authority is required to be obtained or made by the Company in connection with the execution and delivery by it of this Amendment and the performance by the Company of its obligations under this Amendment, except such as have been obtained or made;

(d) as of the date hereof and after giving effect to this Amendment, no Default or Event of Default has occurred and is continuing;

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(e) all the representations and warranties contained in Article V of the Note Purchase Agreement are true and correct in all material respects with the same force and effect as if made by the Company on and as of the date hereof, except as set forth on Schedule A hereto and as modified pursuant to Exhibits B and C hereto; and none of the Material Subsidiaries listed on Schedule 1 to the Note Purchase Agreement are Subsidiaries of Otter Tail Power Company.

ARTICLE III

CONDITIONS TO EFFECTIVENESS OF THIS AMENDMENT

Section 3.1. Conditions Precedent. The amendments contained in this Amendment shall become effective (the “Effective Time”) upon delivery by the Company of, and compliance by the Company with, the following:

(a) executed counterparts of this Amendment, duly executed by the Company and Cascade, shall have been delivered to Cascade;

(b) (i) the representations and warranties of the Company set forth in Section 2.1 hereof are true and correct on and with respect to the date hereof and at the Effective Time, and (ii) the Company shall have complied with all of the obligations contained in this Amendment;

(c) no Default or Event of Default under the Note Purchase Agreement has occurred and is continuing;

(d) Cascade shall have received the favorable opinions of counsel to the Company as to the matters set forth in Sections 2.1(a), 2.1(b) and 2.1(c) hereof, which opinions shall be in form and substance satisfactory to Cascade and shall cover the matters set forth in Exhibit A to this Amendment attached hereto; and

(e) the Company shall have paid the reasonable fees, charges and disbursements of Cascade’s special counsel, Cleary Gottlieb Steen & Hamilton LLP, in connection with the negotiation, preparation, approval, execution and delivery of this Amendment to the extent reflected in a statement of such counsel rendered to Cascade and delivered to the Company.

ARTICLE IV

PAYMENT OF NOTEHOLDER’S COUNSEL FEES AND EXPENSES

Section 4.1. The Company agrees to pay upon demand, the reasonable fees and expenses of Cleary Gottlieb Steen & Hamilton LLP, counsel to Cascade, in connection with the negotiation, preparation, approval, execution and delivery of this Amendment.

ARTICLE V

NOTEHOLDER REPRESENTATIONS AND WARRANTIES

Section 5.1. Cascade represents that it holds all of the outstanding principal amount of the Note.

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ARTICLE VI

MISCELLANEOUS

Section 6.1. This Amendment shall be construed in connection with and as part of the Note Purchase Agreement, and except as modified and expressly amended by this Amendment, all terms, conditions and covenants contained in the Note Purchase Agreement and the Note are hereby ratified and shall be and remain in full force and effect.

Section 6.2. Any and all notices, requests, certificates and other instruments executed and delivered after the execution and delivery of this Amendment may refer to the Note Purchase Agreement without making specific reference to this Amendment but nevertheless all such references shall include this Amendment unless the context otherwise requires.

Section 6.3. The descriptive headings of the various Sections or parts of this Amendment are for convenience only and shall not affect the meaning or construction of any of the provisions hereof.

Section 6.4. This Amendment shall be governed by and construed in accordance with New York law.

Section 6.5. The execution hereof by the parties shall constitute a contract between the Company and Cascade for the uses and purposes hereinabove set forth, and this Amendment may be executed in any number of counterparts, each executed counterpart constituting an original, but all together only one agreement.

[Remainder of page intentionally left blank]

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The foregoing is hereby agreed to as of the date hereof.

(signature page to Amendment No. 5

to Note Purchase Agreement)

SCHEDULE A

(to Amendment No. 5 to Note Purchase Agreement)

Exceptions to Representations and Warranties

EXHIBIT A

(to Amendment No. 5 to Note Purchase Agreement)

DESCRIPTION OF OPINION OF COUNSEL

[See attached]

December 12, 2011

Cascade Investment, L.L.C.

2365 Carillon Point

Kirkland, Washington 98033

Ladies and Gentlemen:

I have acted as counsel to Otter Tail Corporation, a Minnesota corporation (the “Company”), in connection with that certain Amendment No. 5 dated as of December 12, 2011, to the Note Purchase Agreement dated as of February 23, 2007 (the “Amendment”), between the Company and Cascade Investment, L.L.C. (“Cascade”), which amends that certain Note Purchase Agreement, dated as of February 23, 2007 (the “Original Note Purchase Agreement”), as amended by a letter agreement dated December 14, 2007 (the “Letter Agreement”), Amendment No. 2 dated as of June 30, 2009 (“Amendment No. 2”), Amendment No. 3 dated as of June 23, 2010 (“Amendment No. 3”) and Amendment No. 4 dated as of July 29, 2010 (“Amendment No. 4”) (the Original Note Purchase Agreement, as amended by the Letter Agreement, Amendment No. 2, Amendment No. 3 and Amendment No. 4, being referred to herein as the “Note Purchase Agreement”), between the Company and Cascade relating to the issuance and sale by the Company of its 8.89% Senior Note due November 30, 2017 in the aggregate principal amount of $50,000,000. This opinion is being delivered to you pursuant to Section 3.1(d) of the Amendment. Capitalized terms used herein, except as otherwise specifically defined herein, are used with the same meaning as defined in the Note Purchase Agreement, as amended by the Amendment.

In connection with this opinion I have examined such documents and reviewed such questions of law as I have considered necessary and appropriate for the purposes of this opinion.

In rendering my opinions set forth below, I have assumed the authenticity of all documents submitted to me as originals, the genuineness of all signatures (other than the signatures of officers of the Company) and the conformity to authentic originals of all documents submitted to me as copies. I also have assumed the legal capacity for all purposes relevant hereto of all natural persons and, with respect to all parties to agreements or instruments relevant hereto other than the Company, that such parties had the requisite power and authority (corporate or otherwise) to execute, deliver and perform such agreements or instruments, that such agreements or instruments have been duly authorized by all requisite action (corporate or otherwise), executed and delivered by such parties and that such agreements or instruments are the valid, binding and enforceable obligations of such parties. I have also assumed that all conditions precedent to the effectiveness of the Amendment have been satisfied or waived contemporaneously with the delivery of this opinion letter. As to questions of fact material to my opinion, I have relied upon representations and certificates of officers and other employees of the Company and its Subsidiaries (in each case known by me to have authority to make such representations and certifications on behalf of the Company), and certificates of public officials.

December 12, 2011

Page 2

Based on the foregoing, I am of the opinion that:

December 12, 2011

Page 3

The opinions expressed above are limited to the laws of the States of Minnesota and North Dakota and the federal laws of the United States. I express no opinion as to the laws of any other jurisdiction.

The foregoing opinions are being furnished to you solely for your benefit (and the benefit of your successors and assigns) and may not be relied upon by, nor may copies be delivered to, any other person without my prior written consent.

Very truly yours,

George A. Koeck

General Counsel and Corporate Secretary

December 12, 2011

Cascade Investment, L.L.C.

2365 Carillon Point

Kirkland, Washington 98033

Ladies and Gentlemen:

We have acted as special counsel to Otter Tail Corporation, a Minnesota corporation (the “Company”), in connection with that certain Amendment No. 5 dated as of December 12, 2011, to the Note Purchase Agreement dated as of February 23, 2007 (the “Amendment”), between the Company and Cascade Investment, L.L.C. (“Cascade”), which amends that certain Note Purchase Agreement, dated as of February 23, 2007 (the “Original Note Purchase Agreement”), as amended by a letter agreement dated December 14, 2007 (the “Letter Agreement”), Amendment No. 2 dated as of June 30, 2009 (“Amendment No. 2”), Amendment No. 3 dated as of June 23, 2010 (“Amendment No. 3”) and Amendment No. 4 dated as of July 29, 2010 (“Amendment No. 4”) (the Original Note Purchase Agreement, as amended by the Letter Agreement, Amendment No. 2, Amendment No. 3 and Amendment No. 4, being referred to herein as the “Note Purchase Agreement”), between the Company and Cascade relating to the issuance and sale by the Company of its 8.89% Senior Note due November 30, 2017 in the aggregate principal amount of $50,000,000 (the “Note”). The Amendment, the Note Purchase Agreement and the Note are referred to herein as the “Transaction Documents.” This opinion is being delivered to you pursuant to Section 3.1(d) of the Amendment. Capitalized terms used herein, except as otherwise specifically defined herein, are used with the same meaning as defined in the Note Purchase Agreement, as amended by the Amendment.

In connection with this opinion, we have examined such documents and reviewed such questions of law as we have considered necessary and appropriate for the purposes of this opinion. As to questions of fact material to the opinions hereinafter expressed, we have, when relevant facts were not independently established by us, relied upon certificates and opinions of the Company and its officers, other independent third-party sources and of public officials, and we have assumed that all such facts are true and correct as of the date of this opinion. We have not independently examined the records of any court or public office in any jurisdiction, and our opinion is subject to matters which examination of such records would reveal.

In rendering the opinions expressed below, we have assumed, with Cascade’s permission and without verification:

December 12, 2011

Page 2

Our opinions expressed below as to certain factual matters are qualified as being limited “to our knowledge” or by other words to the same or similar effect. Such words, as used herein, mean that prior to or during the course of this firm’s representation of the Company in connection with the specific transactions contemplated by the Note Purchase Agreement, as amended by the Amendment, no contrary information came to the attention (but not including any constructive or imputed notice) of the attorneys currently with our firm who have given substantive attention to such matters on behalf of the Company. In rendering such opinions, we have not conducted any independent investigation of the Company or any of its Subsidiaries, consulted with other attorneys in our firm with respect to the matters covered thereby, or reviewed any of our prior files involving the Company or any of its Subsidiaries. Finally, no inference as to our knowledge with respect to the factual matters upon which we have so qualified our opinions should be drawn from the fact of our representation of the Company.

December 12, 2011

Page 3

Based on the foregoing, we are of the opinion that:

1. The Amendment constitutes the legal, valid and binding agreement of the Company enforceable against the Company in accordance with its terms.

2. Each of the Note Purchase Agreement, as amended by the Amendment, and the Note constitutes the legal, valid and binding agreement of the Company enforceable against the Company in accordance with its terms.

3. No approval or consent of, filing with, notification to, or waiver or exemption from, any Minnesota, New York or federal governmental authority (other than any Minnesota, New York or federal governmental authority regulating public utilities or public utility holding companies, as to which we express no opinion) is required to be obtained or made by the Company in connection with its execution and delivery of the Amendment or the performance by the Company of its obligations pursuant to the Note Purchase Agreement, as amended by the Amendment, except for such approvals, consents, filings, notifications, waivers or exemptions as have been obtained or made and except that we express no opinion regarding any federal securities laws (other than as provided in our opinion in paragraph 4, below), or the securities or “Blue Sky” laws of any state.

4. The Company is not an “investment company” as such term is defined in the Investment Company Act of 1940, as amended, and to our knowledge, based solely on our review of the statements of beneficial ownership of the Company’s stock filed as of the date hereof with the SEC, and in reliance upon certificates of officers of the Company, the Company is not “controlled” by an “investment company” within the meaning of the Investment Company Act of 1940, as amended.

The opinions set forth above are subject to the following qualifications and exceptions:

(a) Our opinions above in paragraphs 1 and 2, above, are subject to the effect of any applicable bankruptcy, insolvency, reorganization, arrangement, moratorium, fraudulent transfer, statutes of limitation or other similar laws and judicial decisions affecting or relating to the rights of creditors generally.

(b) Our opinions above in paragraphs 1 and 2, above, are subject to the effect of general principles of equity, including without limitation, concepts of materiality, reasonableness, good faith and fair dealing, estoppel, election of remedies and other similar doctrines affecting the enforceability of agreements generally (regardless of whether considered in a proceeding in equity or at law). In addition, the availability of specific performance, injunctive relief, the appointment of a receiver, marshalling of assets, stay or other equitable remedies is subject to the discretion of the tribunal before which any proceeding therefor may be brought.

December 12, 2011

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(c) We express no opinion as to the enforceability of provisions in the Transaction Documents to the extent they contain obligations of the Company to pay any early termination fee, prepayment premium, default interest rate or other form of liquidated damages if the payment of such fee, premium, interest rate or damages may be construed as unreasonable in relation to the actual damages or disproportionate to actual damages suffered by the Purchaser as a result of such termination, prepayment or default.

(d) We express no opinion as to the enforceability of the 2001 Note Purchase Agreement and any Credit Agreement which may be deemed to be incorporated by reference into the Note Purchase Agreement pursuant to Section 10.7 of the Note Purchase Agreement or as to the effect such incorporation may have on the enforceability of the Note Purchase Agreement or the Note.

(e) We express no opinion as to the validity, binding effect or enforceability of (i) any provision of the Transaction Documents related to choice of law, forum selection or submission to jurisdiction (including, without limitation, any express or implied waiver of any objection to venue in any court or of any objection that a court is an inconvenient forum), or any provision which purports to confer jurisdiction upon any court or other tribunal, to the extent that the validity, binding effect or enforceability of any such provision is to be determined by any court other than a court of the State of New York, (ii) waivers by the Company of any statutory or constitutional rights, procedural rights, defenses or remedies, including without limitation any waiver by the Company of the right to trial by jury, or the right to recover certain types of damages, or the right to impose counterclaims, or of statutes of limitation or the tolling thereof, (iii) provisions which excuse any person or entity from liability for, or require any person or entity to indemnify any person or entity against, the indemnified person’s or entity’s gross negligence or willful misconduct, or any other indemnification agreement which may be contrary to public policy, (iv) cumulative remedies to the extent such cumulative remedies purport to compensate, or would have the effect of compensating, the party entitled to the benefits thereof in an amount in excess of the actual loss suffered by such party, (v) provisions in the Transaction Documents to the effect that terms thereof may not be waived or modified orally or by course of conduct or that one or more waivers may not under certain circumstances constitute a wavier of other matters of the same kind, or (vi) terms purporting to establish evidentiary standards.

(f) We express no opinion as to compliance or the effect of noncompliance by Cascade with any state or federal laws or regulations applicable to Cascade in connection with the transactions described in the Transaction Documents.

(g) In rendering our opinions in paragraph 3, above, we are only opining as to consents, approvals, authorizations, registrations and filings necessary for Company to execute, deliver and perform its obligations under the Transaction Documents, and we express no opinion with respect to any consent, approval, authorization from, or any registration, declaration or filing with, any governmental authority or agency required generally in connection with the day-to-day business or operations of the Company.

December 12, 2011

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(h) Without limiting clause (g), above, we express no opinion herein with respect to the effect of any land use, safety, hazardous material, environmental or similar law, or any local or regional law.

Except as set forth in paragraph 3, above, we express no opinion as to compliance or the effect of noncompliance by the Company with any laws or regulations applicable to the Company in connection with the transactions described in the Transaction Documents.

The opinions expressed above are limited to the laws of the States of Minnesota and New York and the federal laws of the United States and we express no opinion as to the laws of any other jurisdiction.

The opinions expressed herein are based on an analysis of existing laws and court decisions and cover certain matters not directly addressed by such authorities. This opinion is solely for the benefit of the addressee hereof in connection with the transaction described in the first paragraph of this letter, may not be relied upon by the addressee hereof for any other purpose, and may not be relied upon or used by, circulated, quoted, or referred to, nor may copies hereof be delivered to, another person for any purpose without our prior written approval, provided, however, that such permitted reliance shall not imply or establish an attorney-client relationship between such relying party and this firm with respect to the Transaction Documents or the transactions contemplated thereby, and such relying party by relying on our opinion disclaims any such attorney-client relationship with respect to the Transaction Documents or the transactions contemplated thereby. We disclaim any obligation to update this opinion letter for events occurring or coming to our attention, or any changes in the law taking effect, after the date hereof.

Very truly yours,

EXHIBIT B

(to Amendment No. 5 to Note Purchase Agreement)

SUBSIDIARIES OF THE COMPANY AND OWNERSHIP OF SUBSIDIARY STOCK

The following sets forth a list of the Company’s Subsidiaries as of December 12, 2011 showing, in each case, as to each Subsidiary, the correct name thereof, the jurisdiction of its organization, and the percentage of shares of each class of its capital stock or similar equity interests outstanding owned by the Company and each other Subsidiary.

Subsidiary Guarantors include:

BTD Manufacturing, Inc.

DMI Industries, Inc.

DMS Health Technologies, Inc.

DMS Imaging, Inc.

Foley Company

Aevenia, Inc. (formerly known as Midwest Construction Services, Inc.)

Northern Pipe Products, Inc.

ShoreMaster, Inc.

Vinyltech Corporation

Varistar Corporation

E.W. Wylie Corporation

T.O. Plastics, Inc.

Miller Welding & Iron Works, Inc.

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EXHIBIT C

(to Amendment No. 5 to Note Purchase Agreement)

The following tables provide a breakdown of the assignment of the Company’s consolidated short-term and long-term debt outstanding as of December 2, 2011 and December 31, 2010:

The obligations of the Company under the Agreement are guaranteed by the Subsidiary Guarantors (as defined therein), and the obligations of the Company under the Credit Agreement (as defined in the Agreement) are guaranteed by Varistar Corporation and certain of its Subsidiaries.