Roeland Schaftenaar's 3-pointer with 1.4 seconds left in regulation sent the game into overtime and the Beavers outscored the Trojans 8-4 in the extra period.
"It feels great," Schaftenaar said. "We had a bunch of close ones but we just couldn't get it. Now to get one against USC...just a great feeling."
Schaftenaar tied his career-high with 18 points; Calvin Haynes added 16. Taj Gibson led Southern Cal (10-4, 1-1) with 15 points.
Oregon State's conference losing streak dated back to a 73-65 home win against Washington on Feb. 22, 2007. Twenty-one of the ensuing losses came in the regular season, and the Beavers also lost their opening-round games of the conference tournament each of the past two seasons.
The Beavers are in their first season under coach Craig Robinson, the brother-in-law of President-elect Barack Obama. Change arrived just two games into their conference season.
"It means a heckuva lot to our program, winning this game at this time," Robinson said. "The great thing about winning a game like this early, it gives you a chance to win another game like this, and that's what we are trying to do."
Southern Cal's Keith Wilkinson made a 3-pointer to open the overtime, but the Trojans didn't score again until Wilkinson's free throw with 4.3 seconds remaining.
Oregon State's Omari Johnson tied the game at 57-57 on a 3-pointer with 2:25 remaining, and the Beavers went ahead for good on Haynes' scoop lay-in with 1:23 left. Haynes added another lay-in with 24 seconds remaining.
USC's Dwight Lewis then missed a potential game-tying 3-pointer.
Oregon State scored the game's first eight points, and a basket from Johnson increased the lead to 12-2. USC replied with an 11-0 run, capped by an outside shot from Wilkinson.
The Trojans led 25-20 at the break and extended the advantage to as many as 15 in the second half before the Beavers staged an unlikely rally punctuated by Schaftenaar's 3-pointer.
"(Roeland) was on today," Haynes said. "I already knew it was going in as soon as it was released from his hands."
The Trojans had a chance to win in the final second of regulation. With the ball at midcourt following a time-out, DeMar DeRozan caught an inbounds pass behind the basket and his shot attempt hit the side of the backboard as time expired.
Southern Cal's second-half collapse included four turnovers in the last five minutes of regulation. The Trojans finished with 18 for the game.
"More than anything, their defense forced turnovers with us late," USC coach Tim Floyd said. "We made some poor decisions against the half-court trap that ultimately cost us."
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