Okla. Stat. tit. 12, § 12-729.5
Determining proper money of claim
A. The money in which the parties to a transaction have agreed
that payment is to be made is the proper money of the claim for
payment.
B. If the parties to a transaction have not otherwise agreed,
the proper money of the claim, as in each case may be appropriate,
is the money:
1. Regularly used between the parties as a matter of usage or
course of dealing;
2. Used at the time of a transaction in international trade, by
trade usage or common practice, for valuing or settling transactions
in the particular commodity or service involved; or
3. In which the loss was ultimately felt or will be incurred by
the party claimant.