O'Neill was educated at the University of Toronto and the Ontario College of Education. She worked as a secondary school teacher from 1958 to 1964, and later served as a trustee on the Carleton Roman Catholic Separate School Board of Education.
She was first elected to the Ontario legislature in the 1987 provincial election, defeating her Progressive Conservative opponent by more than 6,000 votes in the riding of Ottawa—Rideau. The Liberals won a landslide majority government in this election under David Peterson, and O'Neill served as parliamentary assistant to the Minister of Education in 1987-88.
The Liberals were upset by the New Democratic Party in the 1990 provincial election, although O'Neill was comfortably re-elected in her own riding. She served as Deputy Opposition Whip from 1990 to 1992, and held a number of official critic positions.
The Progressive Conservatives won a majority government in the 1995 provincial election, and O'Neill lost her riding to Progressive Conservative Garry Guzzo by 1,523 votes. She has not returned to politics since this time. Presently she is with Elections Canada as the Federal Returning Officer for the electoral district of Ottawa West Nepean.