Okla. Stat. tit. 12A § 3-117
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.
Okla. Stat. tit. 12A, § 3-117
Oklahoma Code Comment
Section 3-117 is a recodification of pre-revision Section 3-119, and makes it clear that negotiable instruments are subject to Oklahoma's existing rules concerning admission of prior or contemporaneous agreements to vary the terms of a writing. Generally speaking, the admission of evidence concerning such agreements will be barred by Oklahoma case law and statutes unless the agreement is in writing. Title 15 O.S. § 137 (1910) provides:
The execution of a contract in writing, whether the law requires it to be written or not, supersedes all the oral negotiations or stipulations concerning its matter, which preceded or accompanied the execution of the instrument.
This statute is often referred to as the "parol evidence rule," and in Oklahoma, its application is two-fold. First, Section 137 provides that any oral negotiations, representations or stipulations are superseded and merged into the terms of the executed written agreement. Amerada Petroleum Corp. V. Tucker, 176 Okla. 289, 55 P.2d 54 (1935). It is important to note that the provision reaches not only express agreements, but also statements or representations made by one of the parties to the instrument, absent accident, mistake or fraud in the inducement. See Wichita Flour Mills Co. v. Guymon Equity Exchange, 150 Okla. 245, I P.2d 657 (1931) (evidence of oral representations, as well as agreements, barred by parol evidence rule). See also Knittel v. Security State Bank, Mooreland, 593 P.2d 92 (Okla. 1979) (later statements, even at variance with terms, can create estoppel); Thompson v. E W. Jones Inc.. 189 Okla. 480,118 P.2d 196 (1941) (the rule creates no bar to proving fraud).
Second, Section 137 has consistently been utilized by Oklahoma courts to prohibit introduction of any parol evidence to interpret, vary, or modify the terms of an unambiguous written instrument. See, e.g., Joe Patrick v. Plummer, 291 P. 501 (Okla. 1930); Hicks v. Simmons, 271 F.2d 875 (10th Cir. 1959); Jones v. Sageeyah Devel., Ltd., 833 P.2d 1235 (Okla. 1992). Whether or not an ambiguity exists in any particular agreement is a question of law to be determined by the court. CMI Corp. v. Gurries 674 F.2d 821 (10th Cir. 1982). If the terms of the agreement are clear and unambiguous, its construction is within the exclusive province of the court. 15 O.S. § 154 (1910); Harrison W. Corp. v. Gulf Oil Co. 662 F.2d 690 (lOth Cir. 1981). This rule has been specifically applied to promissory notes. See White v. Oliver, 173 Okla. 559, 49 P.2d 147 (1935); Posey v. Citizens' State Bank, 93 Okla. 266, 220 P. 628 (1923).
As a general proposition, Section 3-117 will only apply in circumstances when the other agreement: (1) is in writing (see Kilpatrick v. Plummer, 145 Okla. 117, 291 P. 501 (1930); Nelson v. Sapulpa State Bank, 88 Okla. 155, 212 P. 309 (1923) (oral evidence excluded)); (2) is referred to (in a manner that does not exclude it from Article 3) in the instrument (see First Nat'l Bank of Stigler v. Howard, 59 Okla. 237, 158 P. 927 (1916)); or (3) the instrument is ambiguous (James v. Citizen's Bank of N Enid, 9 Okla. 546, 60 P. 290 (1900)).