O'Sullivan played baseball for Valhalla High School (El Cajon, California), near San Diego.

O'Sullivan played in 2008 for the Rancho Cucamonga Quakes of the California League (Advanced Class A), and was 16–8 in 158.0 innings with a 4.73 ERA; he was a post-season All-Star pick.

[1] O'Sullivan made his major league debut against the San Francisco Giants on June 16, 2009, at AT&T Park, making a spot-start in replacement of the injured Ervin Santana.

[3][4] After being reassigned to the AAA level, on July 28, 2009, O'Sullivan pitched a no-hitter for the Bees against the Sacramento River Cats.

In April 2012, O'Sullivan failed to make the major league roster and was placed on waivers by the Kansas City Royals.

No other major league club acquired his contract and he was subsequently assigned outright to the Royals' AAA team, the Omaha Storm Chasers.

[7] On June 21, 2012, O'Sullivan was traded to the Toronto Blue Jays for cash considerations and assigned to the Triple-A Las Vegas 51s.

On December 18, 2013, the Philadelphia Phillies announced that they had signed O’Sullivan to a minor league contract that included a spring training invitation.

On April 12, 2015, he was selected to the active roster, but was placed on the disabled list a week and a half later with left knee tendinitis.

He gave up six earned runs in 4.1 innings pitched against Houston in a 10–9 Red Sox victory, but he was designated for assignment by the team on May 15.

[19] On July 3, O’Sullivan was re-added to the active roster, but hit the disabled list with left knee tendinitis a week later.

[24] On March 17, 2018, O'Sullivan signed with the Southern Maryland Blue Crabs of the Atlantic League of Professional Baseball.