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) September 13, 2012

(Date of earliest event reported) September 10, 2012

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

(Zip code)

(918) 588-7000

(Registrant’s telephone number, including area code)

Not Applicable

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

Underwriting Agreement

On September 10, 2012, ONEOK Partners, L.P., a Delaware limited partnership (the “Partnership”), and ONEOK Partners Intermediate Limited Partnership, a Delaware limited partnership and a wholly-owned subsidiary of the Partnership (the “Guarantor”), entered into an underwriting agreement (the “Underwriting Agreement”) with RBS Securities Inc., Mitsubishi UFJ Securities (USA), Inc., and U.S. Bancorp Investments, Inc., as representatives (the “Representatives”) of the several underwriters named therein (the “Underwriters”), with respect to the issuance and sale by the Partnership of (i) $400 million aggregate principal amount of the Partnership’s 2.000% senior notes due 2017 (the “2017 Notes”) and (ii) $900 million aggregate principal amount of the Partnership’s 3.375% senior notes due 2022 (the “2022 Notes” and collectively with the 2017 Notes, the “Notes”).

The Underwriting Agreement contains customary representations, warranties and agreements by the Partnership and the Guarantor, and customary conditions to closing, indemnification obligations of both the Partnership and the Guarantor, on the one hand, and the Underwriters, on the other hand, including for liabilities under the Securities Act of 1933, as amended (the “Act”), obligations of the parties and termination provisions. The Partnership also agreed not to offer or sell certain debt securities until October 10, 2012 without the prior written consent of the Representatives. The foregoing description of the Underwriting Agreement is qualified in its entirety by reference to such Underwriting Agreement, a copy of which is filed herewith as Exhibit 1.1 and is incorporated herein by reference.

The Partnership registered the sale of the Notes with the Securities and Exchange Commission (the “Commission”) pursuant to a Registration Statement on Form S-3 (Registration No. 333-183287) filed on August 13, 2012 (the “Registration Statement”).

On September 13, 2012, the Partnership completed the underwritten public offering (the “Offering”) of the Notes. The Partnership intends to use the net proceeds from the Offering of approximately $1.29 billion, after deducting underwriting discounts and offering expenses, to repay amounts outstanding under its $1.2 billion commercial paper program, and for general partnership purposes, including but not limited to capital expenditures.

Indenture, Supplemental Indenture and Notes

The terms of the Notes are governed by the Indenture, dated as of September 25, 2006 (the “Indenture”), between the Partnership and Wells Fargo Bank, N.A., as trustee (the “Trustee”), as supplemented by the Eighth Supplemental Indenture with respect to the 2017 Notes (the “Eighth Supplemental Indenture”) and the Ninth Supplemental Indenture with respect to the 2022 Notes (the “Ninth Supplemental Indenture”), each of which is dated September 13, 2012.

The Indenture was filed as Exhibit 4.1 to a Form 8-K filed with the Securities and Exchange Commission on September 26, 2006 and is incorporated herein by reference. The Eighth Supplemental Indenture and the Ninth Supplemental Indenture are each filed herewith as Exhibits 4.2 and 4.3, respectively, and are each incorporated herein by reference. The form of the 2017 Notes and the 2022 Notes are each filed herewith as Exhibits 4.4 and 4.5, respectively, and are each incorporated herein by reference. In addition, the legal opinion related to the 2017 Notes, the 2022 Notes and the Guarantees related thereto is attached hereto as Exhibit 5.1 and is incorporated herein by reference.

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Affiliations

The Underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, principal investment, hedging, financing and brokerage activities. The Underwriters and their respective affiliates have provided in the past and may provide from time to time in the future certain commercial banking, financial advisory, investment banking and other services for the Partnership for which they will receive customary fees. Affiliates of the Representatives and certain of the other Underwriters are lenders under the Partnership’s $1.2 billion revolving credit agreement. Certain of the Underwriters and their respective affiliates may from time to time act as dealers under the Partnership’s $1.2 billion commercial paper program. As is described above, some of the net proceeds of this offering may be used to pay down borrowings under the Partnership’s $1.2 billion commercial paper program.

The information included under “Indenture, Supplemental Indenture and Notes” in Item 1.01 above is incorporated herein by reference.

The Partnership issued a news release on September 10, 2012, attached hereto as Exhibit 99.1, announcing the pricing of the Notes. This information is not deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, and is not incorporated by reference into any registration statement under the Act.

(d) Exhibits

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SIGNATURE

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

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EXHIBIT INDEX

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