O'Sullivan was part of the regular first eleven for Plymouth Argyle. At the time Plymouth were placed low in the Nationwide Division 3, he along with Paul McGregor and Martin Phillips were considered the most consistent players at the time. But in the summer of 2000 he headed to Australia to continue his playing career. Following the demise of the old National Soccer League competition and the creation of the A-League, O'Sullivan signed with Central Coast Mariners in late 2004 to play in the newly-formed A-League. At the club, he was a favourite among fans for his tenacity and energy on the field. After an inconsistent 2006–07 season with The Mariners, during which he was often deployed as a right-back, instead of his preferred right-midfield position, the hard-working midfielder/defender was released. Although there is some controversy surrounding Lawrie McKinna's decision to release O'Sullivan, it was believed by many in the Mariners camp that he was past his best and for the club to continue to grow in a developing league, his release was necessary.
After being released by Central Coast Mariners, O'Sullivan had spent months trying to find a spot in another football team to continue his playing career, but was not successful. He holds an Asian Federation Advance Level 'B' coaching licence and in July will graduate from Sydney University with a master's degree in education, specialising in coaching. On Friday, 18 May 2007, O'Sullivan was named Assistant Coach for A-League club Wellington Phoenix on a two-year deal. He worked alongside Ricki Herbert, Luciano Trani and former Celtic goalkeeper Jonathan Gould. On Thursday, 21 November 2013, O'Sullivan was named Assistant Coach for the Central Coast Mariners on a contract until the end of the 2014–15 A-League season, working alongside newly appointed coach Phil Moss. Following the immediate departure of Central Coast Mariner's Head CoachPaul Okon, O'Sullivan was named as the caretaker coach until the end of the 2017–18 A-League season. On Thursday, 16 My 2019, O'Sullivan was appointed as new coach for the remainder of the 2019 National Premier Leagues NSW 2 season with the Bonnyrigg White Eagles FC. To begin the 2019 CAS Football season, O'Sullivan was appointed Head Coach of the Barker College 1st XI, leading them to the title.
