O'Neill studied performance and worked as an actor in England, Europe and the United States before studying theatre writing at the Victorian College of the Arts. She also travelled extensively through South America, Asia and Africa. O'Neill has engaged in performance as much as writing, working with travelling theatre companies. Her one-person show 'Kwaheri Africa' which she performed at La Mama, draws heavily on her travel experience. Her plays include 'Ariadne & Sharon' and 'Scum'; she has also written a screenplay, 'Strike Me Lucky'.

In 1998 O'Neill was awarded a Churchill Fellowship which took her to the United States, the United Kingdom, France and Madagascar. She completed a master's degree in playwrighting at Birmingham University before working in Dublin, Ireland as a writer at the offshore campus of the New York University. During this time she taught English and drama at an asylum-seeker hostel in Western Ireland. In 2001 she was awarded an International Artist Exchange between Britain and Australia to research and write her play, 'Madagascar' which was performed by the Dublin Theatre Works Company in 2004. The New York University commissioned 'Best Possible World', which was performed at The Mint Theatre in Dublin and won the 2003 R E Ross Trust Playwrights Award. O'Neill was an affiliated writer for the Melbourne Theatre Company in 2004 and received an Australian Council Grant from the Literature Board to develop her play 'Our Man Jaman', which was nominated for a Wal Cherry Script of the Year Award. 'Mostar' was shortlisted for the 2005 Patrick White Playwrights' Award. In the same year , 'Best Possible World' was produced as 'Stalking Matilda' , a mystery drama about asylum seekers, at Theatreworks in Melbourne.