He also spent time with 10 other NFL teams: the Green Bay Packers, Chicago Bears, Minnesota Vikings, New England Patriots, Carolina Panthers, Detroit Lions, San Francisco 49ers, Cincinnati Bengals, San Diego Chargers, and Oakland Raiders.

O'Sullivan spent the final season of his professional career as a backup for the Saskatchewan Roughriders of the CFL.

In football, he was a two-time All-Metro League selection and led his school to a Sac-Joaquin Section crown in 1995.

[5] He was also named an Honorable Mention All-American by Don Hansen's Football Gazette and was voted most improved player by his teammates in 1999.

[5][8][9] As a junior in 2000, he threw for 3,679 yards and a school single-season record 38 touchdowns passes as the Aggies advanced to the Division II semifinals, where they lost to Bloomsburg by a score of 58–48.

[5][10][11] O'Sullivan had a 193.85 passer rating and was the only Division II player to receive a Heisman Trophy vote that season.

[5][8] As a senior in 2001, he completed 255 of 406 passes for 32 touchdowns and a school single-season record 3,826 yards as the Aggies advanced to the Division II semifinals, where they lost to North Dakota by a score of 14–2.

[34][36] He started eight games for the Galaxy in 2004, completing 120 of 196 passes for 1,527 yards, ten touchdowns and five interceptions with a record of 6–2 as the starter.

[36][39] The Galaxy finished the year with a 7–3 regular season record, which granted them a berth in World Bowl XII.

[40] In World Bowl XII, against the Berlin Thunder, he completed 19 of 33 passes for 210 yards and 3 touchdowns with 2 interceptions in the 30–24 loss.

[5][38] O'Sullivan and a second round pick in the 2005 NFL draft were traded to the Green Bay Packers for Mike McKenzie on October 4, 2004.

[29][34][47] He was signed off the Bears' practice squad by the Minnesota Vikings on November 8, 2005, after Daunte Culpepper suffered a season-ending injury.

[36][53] On June 21, 2007, O'Sullivan and Cologne Centurions running back Derrick Ross were named the 2007 NFL Europa Co-Offensive MVPs.

[53] He started all ten of the team's games, leading them to a 7–3 record and a berth in World Bowl XV, which the Galaxy lost to the Hamburg Sea Devils by a score of 37–28.

[5][59] Due to injuries to Detroit's other two quarterbacks, Dan Orlovsky and starter Jon Kitna, O'Sullivan played the Lions' final two 2007 preseason games in their entirety.

[5][61][62] On September 16, Kitna suffered a concussion early in the second quarter against the Vikings and was relieved by O'Sullivan, who was making his second career regular season appearance.

[44][63] He then completed 13 of 24 passes for 148 yards, two interceptions and his first career touchdown, which was a seven-yarder to Calvin Johnson, before Kitna returned to the game midway through the fourth quarter.

[63] O'Sullivan played in four games for the Lions in 2007, recording season passing totals of 13 completions, 26 attempts, 148 yards, one touchdown, and two interceptions.

[45][65][66][67] On August 12, head coach Mike Nolan appeared on a local radio show and announced that O'Sullivan had moved into the lead for the 49ers starting quarterback job.

[68][70] On October 26, O'Sullivan, after throwing an interception that was returned for a touchdown, was benched in favor of Shaun Hill by new head coach Mike Singletary during the second quarter of a game against the Seahawks.

[77][78] An unrestricted free agent in the 2009 offseason, O'Sullivan signed a two-year contract with the Cincinnati Bengals in March 2009.

[68] O'Sullivan was signed by the Oakland Raiders on December 2, 2010, after quarterback Bruce Gradkowski was placed on season-ending injured reserve.

[91] He was the team's third-string quarterback for most of the season, behind starter Darian Durant and rookie Drew Willy.

[96] In 2018, O'Sullivan started the YouTube channel "The QB School", where he analyzes game tape of college and pro quarterbacks.

[97][98] He was the head football coach of Patrick Henry High School in San Diego from 2019 to 2021, accruing a 20–10 record.