Okla. Stat. tit. 12, § 2405

A. Where evidence of a person's character or trait of character is admissible, proof may be by testimony as to reputation or by testimony in the form of opinion. Inquiry is allowable on cross- examination into relevant specific instances of conduct. B. In cases in which a person's character or a trait of character is an essential element of a charge, claim or defense, proof may be made of specific instances of his conduct.