Okla. Stat. tit. 12A, § 12A-3-117: Other Agreements Affecting Instrument

OTHER AGREEMENTS AFFECTING INSTRUMENT

Subject to applicable law regarding exclusion of proof of

contemporaneous or previous agreements, the obligation of a party to

an instrument to pay the instrument may be modified, supplemented,

or nullified by a separate agreement of the obligor and a person

entitled to enforce the instrument, if the instrument is issued or

the obligation is incurred in reliance on the agreement or as part

of the same transaction giving rise to the agreement. To the extent

an obligation is modified, supplemented, or nullified by an

agreement under this section, the agreement is a defense to the

obligation.