The announcement Monday was the culmination of the fall camp competition between Mannion and Cody Vaz, who both played at quarterback last season. Both players shared first-team reps for the past several weeks.
Mannion started Oregon State's first four games last season but injured his left knee and required arthroscopic surgery. Vaz stepped in and won the next two games, giving the Beavers their best start since they also went 6-0 in 1907. Oregon State finished a better-than-expected 9-4 and earned a trip to the Alamo Bowl, where it lost to Texas 31-27.
"Sean's attributes are obviously his ability to get the ball just about wherever you want on the field," Riley told reporters gathered after practice on Monday. "Along with his knowledge about where we're going and what he should do with the ball."
Mannion threw for 2,446 yards and 15 touchdowns with eight starts in 10 appearances, while Vaz, hampered late in the season by a left ankle injury, threw for 1,480 yards and 11 touchdowns in seven games with five starts.
Riley said he did not know yet whether both quarterbacks would play in the opener. He said Vaz took the news well.
"That's never fun. I was really proud of his response," Riley said. "I know he'll be ready when called upon."