O’Sullivan is educated locally at Mount Carmel school. After completing a BA at University College Dublin (UCD), she then goes on to work as an English and History teacher and guidance counsellor in a secondary school in Baldoyle, a position she holds for thirty years.
O’Sullivan is a member of Tony Gregory‘s local political organisation in the 1970s, first canvassing for him and later serving as his election agent. She is co-opted onto Dublin City Council for the North Inner City local electoral area from September 2008 to June 2009, after the retirement of Mick Rafferty. After the death of Tony Gregory, she wins the resulting by-election which is held on the same day as the local elections where she also wins a seat on Dublin City Council, for the North Inner City local electoral area. Marie Metcalfe is co-opted to take the seat due to the dual mandate rule. Subsequently Anna Quigley replaces Metcalfe on Dublin City Council, who is in turn replaced by Mel MacGiobúin in March 2014. MacGiobúin fails to be elected at the local elections held in May.
O’Sullivan is re-elected to the Dáil at the 2011 Irish general election. She joins the Dáil technical group which gives independents and minor parties more speaking time in Dáil debates. She describes a proposal for political gender quota legislation as “tokenistic” and that women are able to get themselves nominated for election.
In December 2015, O’Sullivan and fellow independent TDs Clare Daly and Mick Wallace each put forward offers of a €5,000 surety for a 23-year-old man being prosecuted under terrorism legislation in the Special Criminal Court in Dublin charged with membership of an illegal dissident republican terrorist organisation.
After the 2016 Irish general election O’Sullivan unsuccessfully stands for election as Ceann Comhairle. She joins a technical group aligned with Independents 4 Change, while remaining outside the Independents 4 Change party. She is criticised by the brother of late TD Tony Gregory, over an allegedly false claim made in her election literature.
On January 16, 2020, O’Sullivan announces she will not be standing in the 2020 Irish general election in February.