Okla. Stat. tit. 12, § 729.7

A. A person may assert a claim in a specified foreign money. If a foreign-money claim is not asserted, the claimant makes the claim in United States dollars. B. An opposing party may allege and prove that a claim, in whole or in part, is in a different money than that asserted by the claimant. C. A person may assert a claim as a defense, set-off, recoupment or counterclaim in any money appropriate for the claim without regard to the money of other claims. D. The determination of the proper money of the claim is a question of law.