Okla. Stat. tit. 15, § 276
In any civil action to collect upon an obligation to repay money after default, the party prevailing on such cause of action shall be awarded a reasonable attorney's fee. This attorney's fee shall be assessed by the court as costs against the losing party.
Laws 1982, HB 1887, c. 256, § 2, eff. October 1, 1982.