Oregon Constitution Art. II § 16

In all elections authorized by this constitution until otherwise provided by law, the person or persons receiving the highest number of votes shall be declared elected, but provision may be made by law for elections by equal proportional representation of all the voters for every office which is filled by the election of two or more persons whose official duties, rights and powers are equal and concurrent. Every qualified elector resident in his precinct and registered as may be required by law, may vote for one person under the title for each office. Provision may be made by law for the voter's direct or indirect expression of his first, second or additional choices among the candidates for any office. For an office which is filled by the election of one person it may be required by law that the person elected shall be the final choice of a majority of the electors voting for candidates for that office. These principles may be applied by law to nominations by political parties and organizations.