The manager, who was not named, admitted to deflating footballs used in the game, according to a report on the school's website. In the report, USC said no coaches, players, staffers or administrators had any knowledge of the manager's actions, which reportedly took place after game officials tested the balls well before kickoff.
The Pac-12 accepted USC's self-discipline on Wednesday and added a $25,000 fine to the football program.
"We acknowledge the Pac-12's reprimand and fine," USC athletic director Pat Haden said. "We regret this incident occurred. It was unacceptable and we apologize for it. I can assure you this will not happen again."
USC coach Lane Kiffin said the compliance department researched the situation and concluded the manager acted alone.
"They don't know why he did (it)," Kiffin said. "(Quarterback) Matt (Barkley) was questioned in this as well, obviously. He had no knowledge of this at all."
Kiffin was "disappointed" by the manager's actions.
"It was a distraction that nobody here knew about," he said Thursday. "It had no effect on the game. If anything it'd have a negative effect ... throwing two different types of balls for our quarterback. I was just disappointed because it was a distraction that we didn't need."
Three balls were found to be in violation of NCAA regulations before the game began, and two more were discovered at halftime, meaning some first-half plays featured slightly deflated footballs.
Under-inflated footballs typically travel at a slower speed than fully inflated balls and are thought to be easier to catch and throw.
"Obviously as a head coach everything falls on you," said Kiffin. "This situation was brought to our attention not 'til this Sunday by the conference, that there was a situation before the game, where a student manager, on the Oregon sidelines, in front of the Oregon team, was deflating footballs. The compliance department obviously did a very thorough check and research of everything involved to make sure that there was no knowledge of any coaches or players knowing anything about this. I don't know why it was done."
Said Oregon coach Chip Kelly on Sirius XM: "I heard about it postgame, but doesn't affect us. We worry -- and our concern -- is what we can control."
USC lost to Oregon, 62-51, after trailing, 34-24, at halftime.
Information from ESPN's Joe Schad and the Associated Press was used in this report.