Connor Halliday's struggles hurt Washington State's upset bid on Saturday.
Halliday, the Cougars' sophomore quarterback, was pulled early in the second half of a 19-6 loss to No. 14 Oregon State after throwing his third interception.
He was replaced by senior Jeff Tuel, who was 11 of 17 for 126 yards.
Halliday finished 9 of 20 for 81 yards. Tuel briefly replaced Halliday in the second quarter, but Leach went back to Halliday to start the second half.
After Halliday had a turnover on the second play, he was gone for the rest of the game.
"I thought he was going to settle down," coach Mike Leach said. "He's going to be a great player, but he wasn't settled down this game."
Tuel also struggled. He was sacked three times and threw an interception to Jordan Poyer that ended the Cougars' hopes. Poyer also picked off Halliday twice.
The Beavers (4-0, 3-0 Pac-12) needed the defensive effort, as the offense — and quarterback Sean Mannion — struggled from the beginning.
Mannion threw three interceptions and finished 25 of 42 for 270 yards. He was sacked three times.
Markus Wheaton had 95 yards receiving and a touchdown. Oregon State has surpassed its win total for the 2011 season and is 4-0 for the first time since 2002.
Marquess Wilson had four catches for 54 yards to lead the Cougars (2-4, 0-3).
Turnovers hurt the Cougars as they continue to implement Leach's pass-heavy offense.
The Cougars had four interceptions and a fumble.
"(The Beavers) were jumping routes because they were playing a lot of man coverage," Tuel said. "They were playing it tight, and honestly, it's just a small place to throw a football."
Tuel began the season as the starter before giving way to Halliday, who has started the past four games. Leach said he might reopen the competition for the job after looking at the film.
"I'm ready to go whenever, if it's off the bench, if it's starting," Halliday said. "Whatever the coach wants."
Mannion was selected the Pac-12 offensive player of the week after throwing for 433 yards last week in a 38-35 win over Arizona.
But he was not sharp to start this game, missing receivers and throwing two interceptions early.
The Beavers had another drive stopped when they failed to convert a fourth-and-2 at the Washington State 24. Oregon State had to settle for Trevor Romaine's two field goals and a 6-3 halftime lead.
Mannion showed more poise in the second half. He led the Beavers down the field for a 75-yard scoring drive to start the third quarter, hitting Wheaton for a 12-yard touchdown pass.
"I'm really proud of the way our team came out in the second half," Mannion said. "I think that says a lot about us."
In the fourth quarter, Mannion led a 13-play, 86-yard drive that Tyler Anderson capped with a 1-yard run. The Beavers were 3 for 3 on third downs during the drive.
"And two of those third-down conversions were major league," said Oregon State coach Mike Riley, who became the school's winningest coach in last week's win at Arizona.
Helped out by two 15-yard Oregon State penalties, Tuel completed 5 of 7 passes on a drive early in the fourth, but the drive ended in an Andrew Forney 32-yard field goal.
"I thought we were pretty disjointed on offense," Leach said. "Oregon State's defense was more physical than our offense."