He was fired on Monday, with the Trojans 7-10 overall and 2-2 in the Pac-12 after splitting their road trip last weekend. USC lost 66-60 at Colorado on Thursday.
"We just didn't win enough last year and this year. That's what this business is," O'Neill told The Associated Press by phone. "We tried to do as well as we could, tried to get the program to the highest level. My goal every day going in there was to try to win a national title."
He had a 48-65 record at the school known primarily for its powerful football program after going 6-26 last year while setting a USC record for losses.
Athletic director Pat Haden said the program needed new leadership.
"Despite a nice road win in our last game, I felt it was best to make a change now, with most of the Pac-12 season still ahead of us, in order to re-energize our team," he said.
O'Neill was hired by Haden's predecessor, Mike Garrett, who said at the time, "Hopefully he's here forever." O'Neill took the USC job after spending a season as an assistant with the Memphis Grizzlies.
"Nothing in this business surprises me," O'Neill said, adding that he and Haden "really didn't have any discussions other than how the team is doing."
Veteran assistant Bob Cantu will serve as interim coach while USC searches for a successor. Cantu took over for O'Neill during a Pac-10 tournament semifinal game against Arizona in 2010-11 when the coach was suspended after getting into a verbal confrontation with a Wildcats booster. He has remained on the staff through four coaching changes and is the longest tenured assistant in the Pac-12.
"Couldn't be more excited and anxious to get started with #coachbobcantu," tweeted senior center James Blaszyk.
The Trojans lost at Colorado before winning at Utah to snap a 14-game road skid over the weekend. USC has lost nine of its last 13 games, which included a five-game skid.
"There's been a lot of challenges in it but I really enjoyed my time," O'Neill told the AP. "I'm thankful for my opportunity. I'm going to cheer for them every day."
Upon arriving at USC in June 2009, O'Neill had to deal with the fallout from an ongoing NCAA investigation and school-imposed sanctions involving star O.J. Mayo, who played one season under Tim Floyd before leaving for the NBA.