Oregon Revised Statutes Revenue and Taxation § 316.417

(1) A return filed before the last day prescribed by law for the filing thereof is considered as filed on the last day. An advance payment of any portion of the tax made at the time the return was filed is considered as made on the last day prescribed by law for the payment of the tax or, if the taxpayer elected to pay the tax in installments, on the last day prescribed for the payment of the first installment. The last day prescribed by law for filing the return or paying the tax shall be determined without regard to any extension of time granted the taxpayer by the Department of Revenue.

(2) ORS 305.820 applies to returns filed by mail or private express carrier and to due dates that fall on a Saturday, Sunday or legal holiday.