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) May 16, 2014

(Date of earliest event reported) May 13, 2014

ONEOK Partners, L.P.

(Exact name of registrant as specified in its charter)

100 West Fifth Street; Tulsa, OK

(Address of principal executive offices)

74103-4298

(Zip code)

(918) 588-7000

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

Item 1.01 Entry into a Material Definitive Agreement

Underwriting Agreement

On May 13, 2014, ONEOK Partners, L.P., a Delaware limited partnership (the “Partnership”), entered into an underwriting agreement (the “Underwriting Agreement”) with Merrill Lynch, Pierce, Fenner & Smith Incorporated, Barclays Capital Inc., Morgan Stanley & Co. LLC, UBS Securities LLC and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein, with respect to the issue and sale by the Partnership of 12,100,000 common units representing limited partner interests in the Partnership in a public offering (together with an option granted to the underwriters to purchase up to an additional 1,815,000 common units, which the underwriters have exercised in full).

The Underwriting Agreement contains customary representations, warranties and agreements by the Partnership and customary conditions to closing, indemnification obligations of the Partnership and the underwriters, including for liabilities under the Securities Act of 1933, as amended (the “Act”), obligations of the parties and termination provisions. The foregoing description of the Underwriting Agreement is qualified in its entirety by reference to such Underwriting Agreement, a copy of which is filed with this Current Report on Form 8-K as Exhibit 1.1 and is incorporated herein by reference.

Relationships

The net proceeds of the public offering is expected to be used for general partnership purposes and to repay amounts outstanding under our commercial paper program. Affiliates of certain of the underwriters are dealers under our $1.2 billion commercial paper program and, to the extent that they or their affiliates hold such commercial paper, will receive their respective share of any repayment by us of amounts outstanding thereunder. Affiliates of certain of the underwriters are also lenders under our $1.7 billion revolving credit agreement. Citigroup Global Markets Inc. is also a manager under our $300 million “at-the-market” equity program. Affiliates of certain of the underwriters hold our common units.

Item 7.01 Regulation FD Disclosure

The Partnership issued a news release on May 13, 2014, attached hereto as Exhibit 99.1, announcing that it had priced the public offering described in Item 1.01 of this Current Report on Form 8-K at $52.94 per common unit. Following completion of the public offering and after giving effect to the underwriters’ option to purchase additional common units, the Partnership will have 173,991,263 common units outstanding and ONEOK, Inc. and a subsidiary which is the sole general partner of the Partnership, will hold an aggregate ownership interest in the Partnership of approximately 38.8 percent. The general partner of the Partnership will contribute approximately $15.0 million to maintain its two percent general partner interest. The information provided in this Item 7.01 is not deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934 and is not incorporated by reference into any Securities Act registration statements.

Item 9.01 Financial Statements and Exhibits

(d) Exhibits

Reference is made to the “Index of Exhibits” following the signature page, which is hereby incorporated into this item.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, hereunto duly authorized.

EXHIBIT INDEX