The loss on Saturday dropped Oregon State's record to 22-4 overall while San Diego improved to 18-10.
The Beavers scored first on a two-run single by Danny Hayes in the third but San Diego answered back with six in the bottom half of the inning.
Dillon Haupt made it 9-2 in favor of the Toreros when he hit a three-run home run over the center field fence in the fourth. Haupt ended the game with four runs batted in.
Michael Conforto had three hits to pace OSU and Hayes and Beau Day each had two. Oregon State collected 13 overall.
Ben Wetzler took the loss for the Beavers after going three innings. He allowed just three hits but gave up six runs to fall to 0-1 this season.
San Diego's starter, P.J. Conlon, did not go the required distance for a win as he worked just four innings. The lefty allowed seven hits and two runs.
The win went to reliever Max MacNabb, who started his day in the fifth. He improved to 3-2 after going 2 1/3 innings, allowing four hits and one run.
Oregon State returns home for a Tuesday night game against Portland. First pitch is slated for 5:35 p.m. PT.
The game can be heard on the Beaver Sports Network with the following stations carrying the game: KEJO 1240-AM in Corvallis/Albany, FOX Sports Radio Portland (620-AM), KICE 940-AM (Bend) and KSKR 1490-AM in Roseburg.
BASEBALL: Beavs lose first series of year
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