"I am so thankful for the fans and alumni at USC and around the nation that have been so supportive of me and my teammates throughout the season," Mayo said in a statement. "I'd like to express my appreciation to my coaches and teammates for their support in one of the most important decisions I've ever made."
The freshman guard signaled his intentions by wearing socks with the NBA logo in USC's 80-67 loss to Kansas State in the first round of the NCAA tournament.
Mayo helped the Trojans to a 21-12 record, giving them back-to-back 20-win seasons for the first time since 2001-02. Their wins included an upset of crosstown-rival UCLA.
He averaged 20.7 points and 4.5 rebounds, and set freshman school records for scoring average, points in a season (684) and 3-pointers (88).
"This comes as no surprise," coach Tim Floyd said. "We are appreciative of everything O.J. did for all of us the year he was with us. We wish him well. I have no doubt that he will be a great professional."