Unless otherwise authorized by the Commission, bills shall be payable immediately upon receipt and past due twelve (12) days after the date of the OSP mailing or after any deferred payment date previously established either by oral or written agreement between an end-user and the OSP. The date after which the bill is past due shall be stated on the bill. If the bill is not paid when past due, the OSP may apply late payment charges on any unpaid balance as provided in its filed, approved tariffs. Payment shall not be considered late if it is received by the due date at an authorized office.
Okla. Admin. Code § 165:57-10-5