Okla. Stat. tit. 15, § 106

Any benefit conferred, or agreed to be conferred upon the promisor, by any other person, to which the promisor is not lawfully entitled, or any prejudice suffered or agreed to be suffered by such person, other than such as he is at the time of consent lawfully bound to suffer, as an inducement to the promisor, is a good consideration for a promise.