The Beavers committed five turnovers, including four in the second half. Colorado State scored after each of Oregon's State turnovers in the game, totaling 27 points.
Stevens completed 26 of 39 passes with only one interception. His primary target, Michael Gallup, had 11 receptions for 134 yards.
Jake Luton, the fourth Oregon State player to start a season opener at that position in as many years, also passed often. He completed 27 of 47 for 304 yards with two touchdowns and three interceptions.
The Beavers' season high in pass attempts last season was 39 against Washington State. Luton attempted 31 passes by halftime.
One of Luton's interceptions resulted in a 44-yard return for a touchdown by linebacker Tre Thomas with 5:39 left in the third quarter, giving Colorado State a 34-20 lead.
After another Luton interception on the next possession, Wyatt Bryan made his third field goal of the game, a 41-yarder, to increase the Rams' lead to 37-20 with 14:42 left in the fourth quarter.
Colorado State outscored Oregon State 13-0 in the second half before the Beavers took advantage of a fumbled punt return with 14:36 left in the fourth quarter and converted the turnover into a touchdown. On third-and-12, Luton hit Noah Togiai with a 25-yard touchdown pass to cut Colorado State's lead to 37-27.
Oregon State's momentum did not last long. Stevens engineered a touchdown drive on Colorado State's ensuing possession, completing three of four passes for 72 yards, including a 48-yard pass to Gallup.
Stevens finished the drive connecting with Detrich Clark for a 20-yard scoring pass that helped put the Rams ahead 44-27.
Colorado State scored its last two touchdowns in the fourth quarter after an Oregon State fumble and interception.
Dalyn Dawkins, a running back who transferred from Purdue, led Colorado State with 67 yards on 15 carries and one touchdown.
Oregon State fullback Ryan Nall, who broke free for a 75-yard touchdown run in the second quarter, finished with 115 yards on 15 carries.