Stephen Thompson Jr. led the Beavers (16-16) with 12 points, while Drew Eubanks and Tres Tinkle each added 11.
Eubanks had nine rebounds and went 5-for-6 from the free throw line. Tinkle extended his streak to 35 consecutive games scoring in double figures, the fifth-longest streak in program history.
Freshmen Ethan Thompson (5 points, 5 assists, 4 rebounds) and Alfred Hollins (7 points, 4 rebounds) were the other Oregon State starters.
USC jumped out to a 13-2 lead, but Oregon State battled back to cut it to 20-16. The lead grew to 37-24 in the second half before the Beavers chipped away to cut it to 41-35 with 10:56 to play.
The Trojans (22-10) answered with a 9-0 run to make it 50-35 and cruised the rest of the way to advance to the semifinals.
Chimezie Metu led USC with 22 points, and Jonah Mathews and Jordan McLaughlin added 13 points apiece.