In 1986 she appeared in the music video for Simply Red's single "Holding Back The Years".
The youngest of six Catholic brothers and sisters [2] born to head-teacher parents,[3] O'Neill grew up in the Midlands,[3] where she was educated at a convent school, and, after being inspired by a sixth-form drama teacher, "fell" into acting.
[4] After training at the Guildhall School of Music and Drama,[1] she starred in the video for Simply Red's "Holding Back the Years" in 1986, then made her professional stage debut in Moving Pictures by Stephen Lowe at the West Yorkshire Playhouse in Leeds in 1985.
She acted with Keith Barron and Annette Crosbie in the 1989 TV series Take Me Home.
O'Neill also played Max and Jack Branning's sister Suzy in the soap opera EastEnders for five months in 2008.