Ariya Crook had 16 points as the Trojans claimed their first Pac-12 title since winning the regular-season crown in 1994. USC (22-12) trailed, 34-23, late in the first half and 36-29 at halftime, but used a press defense to quickly make up the deficit in the second half.
Deven Hunter led Oregon State (23-10) with 19 points.
USC was already the first team in tournament history to win three games just to reach the final. It capped its four-wins-in-four-days streak with a defensive effort in the second half that flustered the Beavers and took away any doubt about its NCAA tournament status.
USC was making its second appearance in the title game, losing to Stanford in 2009. It was the first time in tournament history that Stanford wasn't involved in the final after the Cardinal lost to the Trojans in the semifinals Saturday.
But USC wasn't done after knocking off the No. 4 team in the country. First-year Coach Cynthia Cooper-Dyke then directed the Trojans to their first NCAA tournament berth since 2006.
Cassie Harberts, who came up big in the closing minutes against Stanford, added 12 points and Brianna Barrett had 11 for the Trojans.
Sydney Wiese added 17 points for Oregon State, which was the first team outside California or Arizona to reach the conference title game.
Oregon State could be headed for the NCAA tournament a season after finishing 12th in the conference, but was unable to claim its first Pac-12 title of any type in women's basketball. Oregon State has not been to the NCAA tournament since 1996.
USC trailed by seven at halftime, but then scored the first nine points of the second half, causing havoc with a press defense Oregon State struggled to break.
The Beavers committed turnovers on four of their first six possessions to start the half. Their five turnovers in the first five minutes of the half nearly surpassed the six they had in the entire first half.
Oregon State struggled to even get into its offense and when it did, the shots didn't fall. The Beavers missed 11 of 15 shots to start the second half and started one for seven on three-pointers.
Ruth Hamblin, Oregon State's 6-foot-6 center, was a nonfactor. She was held scoreless for nearly 13 minutes of the second half before hitting a pair of free throws with 7:02 left to get the Beavers within 10.
The Beavers went nearly nine minutes without a field goal between Wiese's three-pointer with 11:53 remaining and Hunter's driving layup with 3:03 left. By the time Hunter scored, the deficit was too much to overcome. Oregon State got within five, but Harberts scored with 1:17 left with one on the shot clock and the Trojans hit free throws to hold on.