You must register to vote by Tuesday, April 24th, 2018, if you want to vote in the Oregon primary.
View the 2018 Election Timeline (PDF) at the Oregon Secretary of State’s Election Division website. (If the link to the Timeline PDF fails, link to the Election Division’s URL and search there.)
If you like to calculate your own election registration deadline:
A new registrant must submit their online registration by 11:59:59 p.m. Pacific Time on the 21st calendar day before an election to be eligible to vote in that election.” [From the Oregon Secretary of State Voting and Elections website.]