Whakaotihia te rerenga e whai ake nei: I whanau mai i Cobh, County Cork, i whakaakona a O'Sullivan i Cobh Community College. I oti i a ia tana tiwhikete whakarere i te tau 1987. Ko te whakataetae nui tuatahi a O'Sullivan o te ao ko te
1990 European Championships i Split, i mutu ai ia i te 11 i te 3000 m. Heoi, i tere te whakapai ake a O'Sullivan i muri mai, a i te 26 o Hanuere o te tau e whai ake nei, i Boston, i whakatauhia e ia he rekoata hou o te ao i roto i te 5000 m o te 15:17.28, neke atu i te 5 hēkona i te rekoata tawhito. I taua wa, kei te ako a O'Sullivan (i whiwhi i tetahi karahipi hakinakina) i te Whare Wananga o Villanova i te US, ana i nga Taakaro Akoranga o te Ao i Sheffield i te Hurae 1991, i wini ia i te 1500 m Tohu Koura i te 4: 2.14, me te tohu hiriwa i te 3000 m i te 8:56.55. I te timatanga o te tau 1992, i mutu ia i te 7 o nga Toa o te Ao Cross Country.