710:50-13-10 The return for the taxable year

If an individual taxpayer files an amended return after filing the original return and before the due date for filing the original return, including any extensions, the amended return constitutes "the return for the taxable year" pursuant to 68 O.S. § 2385.13. The amount shown on the amended return is used to determine the amount of underpayment. If the amended return is filed after the due date it is not "the return for the taxable year" for purposes of Section 2385.13. In that case, the tax shown on the amended return will not be used to compute the amount of underpayment.