Born in Rusholme, Manchester, O'Brien left Trinity Church of England High School with two GCSEs. She starred in the drama Children's Ward in 1997, The Cops in 1998 and Clocking Off in 2000, playing the on-screen sister of actor Jack P Shepherd who later became her on-screen brother David in Coronation Street.

She joined Coronation Street on 31 October 1999, taking over the role of Sarah-Louise Platt from Lynsay King. At the age of sixteen, she was three years older than her character. Sarah's pregnancy with baby Bethany won O'Brien Best Storyline at the British Soap Awards.

She has also won best newcomer at the National Television Awards. O'Brien revealed in the British press on 4 April 2007 that she would be leaving Coronation Street before the end of the year.

Her first role after leaving Coronation Street was in the 2007 pantomime at Manchester Opera House, where she played the title character of Cinderella. O'Brien has expressed interest in doing televised dramas and theatre work.