There is an Amended check-box on the top of Form 511.
When should I file an amended return?
Generally, to claim a refund, your amended return must be filed within three years from the date tax, penalty and interest were paid. For most taxpayers, the three year period begins on the original due date of the Oklahoma tax return. Estimated tax and withholdings are deemed paid on the original due date (excluding extensions). If your federal return for any year is changed, an amended Oklahoma return shall be filed within one year. If you amend your federal return, it is recommended you obtain confirmation the IRS approved your federal amendment before filing your amended Oklahoma return. Filing an amended Oklahoma return without such IRS confirmation may delay the processing of your return; however, this may be necessary to avoid the expiration of the statute of limitation. File a separate amended return for each year you are amending. No amended return may encompass more than one single year. Mail each years amended return in a separate envelope. Do not enclose amendments from different years in the same envelope. If you discover you have made an error only on your Oklahoma return, we may be able to help you correct the form instead of filing an amended return. For additional information, contact our Taxpayer Assistance Division at (405) 521-3160 or in state toll free at (800) 522-8165. Press 0 to speak with a representative.
Place an “X” in the Amended Return check-box at the top of Form 511, page 1. Complete the amended return. Enter any amount(s) paid with the original return plus any amount(s) paid after it was filed on line 29. Enter any refund previously received or overpayment applied on line 31. Complete Schedule 511-H “Amended Return Information” on Form 511, page 5.
Please see oklahoma.gov for more information.