Okla. Stat. tit. 58, § 550

In rendering his account, the executor or administrator must, upon request of the court or an interested party, file vouchers or other proof of payment for all charges, debts, claims, and expenses which he has paid, which must remain in the court; and he may be examined on oath touching such payments, and also touching any property and effects of the decedent, and the disposition thereof. When any voucher or other proof of payment is required for other purposes, it may be withdrawn on leaving a certified copy on file; if a voucher or other proof of payment is lost, or for other good reason cannot be produced on the settlement, the payment may be proved by the oath of any competent witness.