O’Sullivan was born in Perth, Western Australia, and later relocated to Sydney to study at the National Institute of Dramatic Art where he graduated in 2003.
His involvement at the 2006 Short and Sweet theatre festival helped boost his profile and he gained numerous other stage roles.
[3] O’Sullivan's first television role was playing Keith Harris, a reporter who was punched in the face by the character Alf Stewart (Ray Meagher) following a helicopter crash, on the soap opera Home and Away.
[4][6] O'Sullivan secured the role of Tom Huppatz in the mini-series Rain Shadow, which aired on ABC TV from 7 October 2007.
[11][7] To prepare for his role in Underbelly, the actor read books written by policemen who were involved with the subject matter of the show.
[12] He then secured a role in the main cast of Nine Network drama Cops LAC playing Senior Constable Nathan Holt.
[18] Next came a guest appearance in the drama A Place to Call Home playing Jack Duncan Snr in the episode titled "The Ghosts of Christmas Past", which aired on 8 June 2014.
[7][26] Notable works include playing Ferdinand and Caliban in Harlos Productions 2007 stagings of Shakespeare's The Tempest and Peter in Dying City.
[27][28] In 2009, O'Sullivan took the role of Craig in the Black Swan State Theatre Company production of The Dark Room, an original Australian play.
[30] In September 2011, the actor starred in a Cat On A Hot Tin Roof at Perth's State Theatre Centre of Western Australia.
[31] From January to February 2012, O'Sullivan appeared as Stanley Saunders in Hillary Bell's The White Divers of Broome at the Heath Ledger Theatre.