Okla. Admin. Code § 710:50-9-2
When an original income tax return has been filed, the statute of limitations for filing a claim for a refund of an overpayment of income tax in Oklahoma is generally three (3) years from the due date of the tax return, including any extensions of time for filing, or two (2) years from the date the tax was paid, whichever is later. If an original return has not been filed, the statute of limitations for filing a claim for a refund of an overpayment of income tax in Oklahoma is two (2) years from the date the tax was paid. A refund that is "barred by statute" cannot be used as payment on any delinquent account or applied to estimated tax. Exceptions to the statute of limitations set out in 710:50-5-13 also apply to certain refund situations. [See: 68 O.S. §2373]
Okla. Admin. Code § 710:50-9-2