Whenever the Uniform Commercial Code creates a "presumption" with respect to a fact, or provides that a fact is "presumed", the trier of fact must find the existence of the fact unless and until evidence is introduced that supports a finding of its nonexistence.
Okla. Stat. tit. 12A, § 1-206
Oklahoma Code Comment
Revised section 1-206 is based on old section 1-201(31). Oklahoma had no statutory definition prior to the UCC.