Okla. Stat. tit. 68 § 2206
Every railroad company using, renting or leasing the freight cars of any freight line company, equipment company, or mercantile company shall, upon making payment to such company for the use, rental, or lease of such cars, withhold from such payment four percent (4%) of the amount constituting the gross revenue in this state from such source of each and every freight car so used, rented or leased.
On or before April 1 of each year, such railroad company shall prepare and file with the Tax Commission a statement under oath showing the amount of such payment for the next preceding twelve-month period ending December 31, and of the amount so withheld by it; provided:
The statement shall be on forms prescribed and furnished by the Tax Commission, and shall contain such information as the Tax Commission may deem necessary.
Such statements shall be accompanied by remittance in full of all taxes withheld by the railroad company from freight line companies, equipment companies and mercantile companies during the next preceding twelve-month period ending December 31 or the period specified in paragraphs 1 and 2 of this section. Each railroad company shall be liable for the withholding and payment on or before April 1 of each year, of four percent (4%) of the gross revenue of each freight line company, equipment company and mercantile company, to the extent that such gross earnings were derived from payments or amounts due from such railroad company.
Each freight line company, equipment company and mercantile company shall be liable for the payment of four percent (4%) of all gross revenue in this state over and above payments and amounts due from railroads, and shall be liable for the payment of any additional taxes which the Commission may find due under its authority to raise or lower the rate to conform to the taxes which would be payable if the cars were taxed on an ad valorem basis.
Okla. Stat. tit. 68, § 2206