Sean Mannion, a redshirt freshman who took over as Oregon State's starter in Game 3, threw a pair of scores in the first half, then connected with Joe Halahuni on a 25-yard touchdown in the third quarter as the Beavers ran off 20 unanswered points.
Mannion finished 26 of 34 and threw another TD in the fourth quarter.
Given time in the pocket due to little pass rush from Washington State's front seven, Mannion picked apart the Cougars secondary. Mannion had touchdown passes of 31 yards to Jordan Jenkins on Oregon State's opening drive, 12 yards to Brandin Cooks, his third-quarter TD to Halahuni and an 18-yard toss to Clayton York in the fourth quarter to push the Beavers lead back to 20.
How much confidence did coach Mike Riley have in Mannion? On a key third-and-1 early in the fourth quarter, the Beavers went play-action and Mannion found James Rodgers for 20 yards.
And when Mannion wasn't finding open receivers, Agnew was gashing the Cougars for healthy chunks of yards. Agnew averaged 4.5 yards per carry and had his second 100-yard game of the season. Oregon State entered ranked 107th in the country at running the ball, but finished with 175 yards rushing.
Trevor Romaine also kicked three short field goals for the Beavers.
Washington State lost quarterback Jeff Tuel at halftime with an apparent injury. He was replaced by Marshall Lobbestael, but the Cougars never stopped the Beavers offense.
Oregon State (2-5, 2-3 Pac-12) never punted until the final 90 seconds of the fourth quarter and would have scored on every drive of the game if not for a first-quarter interception by Washington State's Damante Horton.
The Cougars (3-4, 1-3) dropped their third straight after a promising 3-1 start to the season and now face the teeth of their schedule with upcoming games at Oregon, California, home for Arizona State and the Apple Cup against rival Washington in the final five weeks of the season. While Washington State has already surpassed its win total from a year ago, the expectation was the Cougars would need to be in bowl contention to earn coach Paul Wulff an extension.
Saturday was considered a must-win for the Cougars to inch closer to that six-win mark.
Instead, it was the Beavers playing with the urgency Washington State lacked.
Some of that urgency could have come from the Beavers seeking payback for a 31-14 setback at the hands of the Cougars last year in Corvallis, a win that might have saved Wulff's job at the time.
Tuel threw for 127 yards and a touchdown, and added another touchdown running, but was sacked three times and took a big hit on an incompletion just before halftime. He was late returning from the locker room at half and was replaced by Lobbestael, the starter for the first five games of the season. Tuel broke his collarbone in the opener against Idaho State - a game Lobbestael started - and returned for last week's 44-14 loss to Stanford.
Lobbestael finished 10 of 20 for 105 yards and was intercepted in the end zone late when only orange and black clad Beavers fans were still around. Marquees Wilson had a 20-yard touchdown catch in the first half.
Saturday was the return of the Cougars annual game in Seattle at the home of the Seattle Seahawks. The Cougars will surrender a home game on campus in Pullman and play either Oregon or Oregon State in Seattle each year.