He attended the University of California, Berkeley, and played for the California Golden Bears football team. In his senior year with the Golden Bears, O'Neal set the NCAA single-season record of four interceptions returned for touchdowns and was recognized as a consensus first-team All-American. He graduated from California with a degree in social welfare in 2000.
Due to O'Neal's speed, former Broncos head coach Mike Shanahan briefly flirted with converting O'Neal to wide receiver in mid-2003. O'Neal had two receptions, for four yards, in two games. O'Neal was also used frequently to return punts and kickoffs on special teams. While with the Broncos, O'Neal tied the record for most interceptions in a game with four.
In April 2004, an unhappy O'Neal and the Broncos' 24th (first round) and 117th (fourth round) spots in the 2004 NFL draft were traded to the Cincinnati Bengals in exchange for Cincinnati's 17th spot in the first round.
In the 2005 season, O'Neal recorded ten interceptions, breaking the franchise record of nine, set by Ken Riley in 1976 and equalling Ty Law as the NFL's interception leader that year. He broke the record on December 18, 2005, against the Detroit Lions.
In early December 2006, O'Neal was arrested on DWI charges. In the 2007 season, O'Neal recorded 52 tackles and one interception.
O'Neal was released by the Bengals on August 30, 2008.