Oregon Revised Statutes Elections § 254.365

(a) The elector is registered as being affiliated with one of the major political parties nominating or electing its candidates for public office at the primary election; or

(b) The elector is registered as not being affiliated with any political party and wishes to vote in the primary election of a major political party that has provided under subsection (3) of this section for a primary election that admits electors not affiliated with any political party.

(3)(a) Not later than the 90th day before the date of the primary election, a major political party may file with the Secretary of State a certified copy of the current party rule allowing an elector not affiliated with any political party to vote in the party's primary election. The party may not repeal the rule as filed during the 90 days before the primary election. The rule shall continue to be effective after the date of the primary election until the party gives written notice to the Secretary of State that the rule has been repealed. Except as provided in paragraph (b) of this subsection, a party rule under this subsection may limit the candidates for whom an elector who is not affiliated with any political party may vote.

(b) The party rule shall allow any elector who is permitted to vote for the most numerous branch of the Legislative Assembly also to vote in federal legislative elections, consistent with section 2, Article I, and the Seventeenth Amendment to the United States Constitution.