Okla. Stat. tit. 68 § 500.43
In the event the tax imposed by this act is not precollected and must be collected from the consumer in accordance with Section 28 of this act, the tax is due and payable by the consumer on the first day of each month for the preceding calendar month, and if not paid on or before the 15th day of the following month, shall be delinquent. The consumer shall file with the Commission, on forms furnished by the Commission, a return verified by affidavit showing in detail the total purchase price of the motor fuel, the number of gallons purchased, the price per gallon, the location of the purchase and any other information the Commission may deem reasonably necessary. With each return, the consumer shall remit to the Commission the amount of tax shown on the return to be due. Reports timely mailed shall be considered timely filed. If a report is not timely filed, interest shall be charged from the date the report should have been filed until the report is actually filed.
Okla. Stat. tit. 68, § 500.43