Exhibit 5.1

ONEOK, Inc.

100 West Fifth Street

Tulsa, Oklahoma 74103

Ladies and Gentlemen:

We have acted as special counsel to ONEOK, Inc., an Oklahoma corporation (the “Company”), ONEOK Partners, L.P., a Delaware limited partnership (“ONEOK Partners”), and ONEOK Partners Intermediate Limited Partnership, a Delaware limited partnership (“ONEOK Partners Intermediate” and, together with ONEOK Partners, the “Guarantors”), in connection with the issuance by the Company of $600 million aggregate principal amount of its 5.850% Senior Notes due 2026 (the “2026 Notes”), $600 million aggregate principal amount of its 6.350% Senior Notes due 2031 (the “2031 Notes”) and $300 million aggregate principal amount of its 7.150% Senior Notes due 2051 (the “2051 Notes” and, together with the 2026 Notes and the 2031 Notes, the “Notes”) and the guarantee of the Notes (the “Guarantee” and, together with the Notes, the “Securities”) by the Guarantors, under the Base Indenture dated as of January 26, 2012, as amended and supplemented (the “Base Indenture”), by and among the Company and U.S. Bank National Association, as trustee (the “Trustee”), as further supplemented by the Sixteenth Supplemental Indenture, dated as of May 7, 2020 (the “Sixteenth Supplemental Indenture”), the Seventeenth Supplemental Indenture, dated as of May 7, 2020 (the “Seventeenth Supplemental Indenture”), and the Eighteenth Supplemental Indenture, dated as of May 7, 2020 (the “Eighteenth Supplemental Indenture” and, together with the Sixteenth Supplemental Indenture and the Seventeenth Supplemental Indenture, the “Supplemental Indentures” and, the Base Indenture as so supplemented, the “Indenture”) and pursuant to a registration statement on Form S-3 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on July 6, 2017 (Registration No. 333-219186) (as so filed, the “Registration Statement”). This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related prospectus, other than as expressly stated herein with respect to the issuance of the Securities.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company, the general partners of each of the Guarantors and others as to factual matters without having independently verified such factual matters. We are opining herein as to the internal laws of the State of New York and the Delaware Revised Uniform Limited Partnership Act, and we express no opinion with respect to the applicability thereto, or the effect thereon, of the laws of any other jurisdiction or, in the case of Delaware, any other laws, or as to any matters of municipal law or the laws of any local agencies within any state.

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, when the Securities have been duly executed, issued, and authenticated in accordance with the terms of the Indenture and delivered against payment therefor in the circumstances contemplated by the underwriting agreement, dated May 4, 2020, among the Company, the Guarantors and Citigroup Global Markets Inc., BofA Securities, Inc., J.P. Morgan Securities LLC, Morgan Stanley & Co. LLC and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein, the Securities will have been duly authorized by all necessary limited partnership action of the Guarantors, and will be legally valid and binding obligations of the Guarantors, enforceable against the Company and the Guarantors in accordance with their terms.

Our opinion is subject to:

With your consent, we have assumed (a) that the Indenture and the Securities (collectively, the “Documents”) have been duly authorized, executed and delivered by the parties thereto other than the Guarantors, (b) that the Documents constitute legally valid and binding obligations of the parties thereto other than the Guarantors, enforceable against each of them in accordance with their respective terms, and (c) that the status of the Documents as legally valid and binding obligations of the parties is not affected by any (i) breaches of, or defaults under, agreements or instruments, (ii) violations of statutes, rules, regulations or court or governmental orders, or (iii) failures to obtain required consents, approvals or authorizations from, or make required registrations, declarations or filings with, governmental authorities.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Company’s Form 8-K dated May 7, 2020 and to the reference to our firm contained in the Prospectus under the heading “Legal.” In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.