Okla. Stat. tit. 52 § 54
Every corporation, joint stock company, partnership or other person, claiming or exercising the right to carry or transport crude oil or petroleum or any of the products thereof, by pipeline for hire or otherwise, within the limits of this state, as allowed by, and upon compliance with the requirements of this article, as owner, lessee, licensee, or by virtue of any other right or claim, which is engaged in the business of purchasing crude oil or petroleum therein, shall be deemed a common purchaser thereof, and shall purchase all of the petroleum in the vicinity of, or which may be reasonably reached by its pipelines, or gathering branches, without discrimination in favor of one producer or one person as against another, and shall fully perform all the duties of a common purchaser; but if it shall be unable to perform the same, or shall be legally excusable from purchasing and transporting all of the petroleum produced, then it shall purchase and transport petroleum from each person and producer ratably, in proportion to the average daily production; and such common purchasers are hereby expressly prohibited from discriminating in price or amount for like grades of oil, or facilities as between producers or persons; and in the event such purchaser is likewise a producer, it is hereby prohibited from discriminating in favor of its own production, or storage, or production or storage in which it may be interested, directly or indirectly in whole or in part, and its own production and storage shall be treated as that of any other person or producer.
Okla. Stat. tit. 52, § 54