Special election to fill vacancy caused by death of John H. Mitchell on December 8th, 1905. John M. Gearin was appointed to replace Mitchell, but did not run in the special election to fill the remainder of the term expiring on March 4, 1907. Gearin would instead run in (and lose) the regular election for a full term beginning March 4, 1907.
This election, along with the simultaneous regular election, was the first use of the popular vote for electing a United States Senator. In 1904, Oregon passed the Direct Primary Law via initiative, which created a non-binding pledge that members of the Legislative Assembly would select the winner of the popular vote as United States Senator. Fredrick Mulkey easily won the popular vote at the general election in June 1906 as there was no Democratic opposition. His selection to serve out the unexpired term was subsequently ratified by the Legislative Assembly on January 22, 1907.
Source for Popular Vote data: Oregon Secretary of State. Abstract of Votes Cast at a General Election held in the State of Oregon on the fourth of June, 1906, for State Officers, United States Senator and Representatives in Congress, Oregon Secretary of State (http://records.sos.state.or.us/ORSOSWebDrawer/Record/8410979) (accessed 15 Sep 2023)