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CORVALLIS, Ore. – Sean Mannion couldn't really explain why he had an off night.
Oregon State's 6-foot-5 quarterback, who is having a career year, threw three uncharacteristic interceptions in Friday night's 31-14 loss to Southern California.
The interceptions doubled his total over the first eight games of the season. And two of them came in the red zone.
"When you play quarterback, you are the one with the ball in your hands and ultimately the one who makes the decisions on where to go with it," he said. "I think I just need to see that and throw it away."
Mannion completed 26 of 45 passes for 277 yards and a touchdown. He went into the game leading the nation with an average of nearly 408 yards passing per game, and he ranked fourth with nearly 34 completions per game.
"I felt fine, it was just a matter of forcing a couple of balls that I should have thrown away," he said. "The two in the red zone especially upset me."
It was the second straight loss for the Beavers (6-3, 4-2 Pac-12) after a six-game winning streak. Oregon State has a bye next weekend before visiting Arizona State.
Javorius Allen ran for 133 yards and three touchdowns, while Marqise Lee returned from a knee injury and had five catches for 105 yards and a score for USC (6-2, 3-2), which became bowl eligible.
Cody Kessler threw for 247 yards and a touchdown and Silas Redd ran for 140 yards for the Trojans, who have won three of their last four games under interim coach Ed Orgeron.
Mannion hit favorite target Brandin Cooks with a 43-yard pass on the opening series and the Beavers were helped by a pass interference call, but Trevor Romaine's 46-yard field goal attempt was blocked.
The Trojans took over on the 29 and, on the first play from scrimmage, Kessler hit Lee with a 71-yard touchdown pass to put USC up 7-0.
Lee, last year's Biletnikoff Award winner as the nation's top receiver, was questionable going into the game because of a left knee injury that has sidelined him for two of the last three games.
The Trojans went up 14-0 on Allen's 9-yard scoring run late in the first quarter.
Oregon State closed the gap with Mannion's 27-yard touchdown pass to Cooks. On the next series, Ryan Murphy intercepted Kessler's pass and ran it back 41-yards to even it at 14 with 9:29 to go in the half.
Allen, a sophomore who had scored two touchdowns this season going into the game, put USC back on top with an 18-yard run and the Trojans led 21-14 at halftime.
Southern California opened the second half with Andre Heidari's 34-yard field goal. Oregon State had a chance to pull closer, but Romaine's 26-yard attempt went wide left.
Allen added a 52-yard touchdown run with 3:25 left in the third quarter to give USC a 31-14 lead.
The Trojans were coming off a 19-3 victory over Utah last Saturday.
Oregon State was notably without senior receiver Kevin Cummings, who injured a wrist sometime during the first half of the loss to Stanford and required surgery. He could be back for a bowl game.
The Beavers also were hit on the opening kickoff when linebacker Joel Skotte collided with a USC player and lay still for several moments before he was helped off the field by trainers. Riley did not have a specific injury report but said Skotte was alert and talking.
The Beavers are known for their success against some very formidable Trojans teams. In 2008, the Beavers were 26-point underdogs when they knocked off then-No. 1 USC 27-21 on a Thursday night in Corvallis. Two years earlier, Oregon State topped then-No. 3 USC at Reser 33-31.
The two teams hadn't met since 2010, when the Beavers downed then-No. 20 USC in Corvallis. Ryan Katz threw for 154 yards and two touchdowns while Jacquizz Rodgers rushed for 128 yards and another score in the 36-7 victory.
USC's last win in Corvallis was a foggy 28-20 victory in the 2004 season when the Trojans went undefeated and beat Oklahoma in the Orange Bowl for the national championship — which was later vacated by the NCAA sanctions over improper player benefits.