Okla. Admin. Code § 260:10-3-6
Amendments to purchase contracts, commonly known as Change Orders, for overshipments of goods accepted by an agency, shall be considered independent from the original contract for purposes of interest entitlement as provided by this chapter except that the clock shall begin with the date the amendment (Change Order) is approved for that portion of the contract only. This section does not preclude a vendor from submitting a proper invoice for goods and services provided in conjunction with the terms of the original contract. It also does not preclude an agency from "marking down" an otherwise proper invoice which includes such overshipment to the amount specified in the original contract, in order to expedite payment to the vendor for the original contract quantity.
Okla. Admin. Code § 260:10-3-6