O'Neil attended Sweet Home High School, where he played linebacker.
O'Neil was signed as an undrafted free agent by the Dallas Cowboys after the 2003 NFL draft.
On September 3, 2005, he was waived because the defense switched to a 3-4 alignment and he wasn't seen as a good fit for the new scheme.
On July 30, 2007, he was placed on the Physically unable to perform list recovering from a sports hernia surgery.
On February 18, 2008, he was signed as a free agent by the New York Giants, after spending a year out of football.