OSU had four players in double figures, led by Devon Collier's 19 points. The sophomore forward connected on all but one of his eight field goal attempts. Joe Burton came off the bench to chip in 15 points, while Ahmad Starks, who hit four three-pointers in the first half, added 13.
Making the win all the more impressive for Oregon State was the fact that Jared Cunningham, the Pac-12's leading scorer, hit just 1-of-8 shots from the field and scored only four points.
The Beavers now move on to face the top-seeded Washington Huskies Thursday afternoon in the quarterfinals. OSU dropped both regular season meetings with the Huskies, most recently falling 75-72 at home on Feb. 12.
Oregon State got off to a sizzling start and raced out to an early 24-8 lead over the Cougars. Washington State answered the bell and responded by shooting 65.4 percent (17-of-26) in the first half to take a 39-38 lead into the locker room.
The teams went back and forth in the second half before Collier and Burton helped the Beavers pull away in the final minutes.
Washington State, which was led in scoring by Brock Motum's 18 points, finished the season at 15-16.