Kammala jumla mai zuwa: An haife shi a Cobh, County Cork, O'Sullivan ya yi karatu a Kwalejin Kwalejin Cobh. Ta kammala takardar shaidarta a shekarar 1987. Babban gasar kasa da kasa ta farko ta O'Sullivan ita ce
1990 Gasar Zakarun Turai a Split, inda ta gama ta 11 a tseren mita 3000. Koyaya, O'Sullivan ya inganta da sauri bayan wannan, kuma a ranar 26 ga Janairu na shekara mai zuwa, a Boston, ta kafa sabon tarihin duniya a cikin gida a cikin mita 5000 na 15:17.28, yana ɗaukar fiye da dakika 5 daga tsohon rikodin. A lokacin, O'Sullivan (wanda ya sami tallafin karatu na wasanni) yana karatun lissafi a Jami'ar Villanova a Amurka, kuma a Wasannin Dalibai na Duniya a Sheffield a watan Yulin 1991, ta lashe lambar zinare a tseren mita 1500 a cikin 4:12.14, da kuma lambar azurfa a tseren mita 3000 a cikin 8:56.55. A farkon 1992, ta gama 7th a Gasar Cin Kofin Duniya.