Oregon Revised Statutes Revenue and Taxation § 316.588

(a) If an amended return is filed on or before the return due date (determined with regard to any extension of time granted to the taxpayer), then the term “return” means the amended return.

(b) If during initial processing of the return the Department of Revenue adjusts the amount of tax due, then the term “tax shown on the return” means the tax as adjusted by the department. This paragraph shall not apply if it is ultimately determined that the adjustment was improper.