Born in Rusholme, Manchester, England, O'Brien left Trinity C.E. High School with nine GCSEs. She starred in the drama "Children's Ward" in 1997, "The Cops" in 1998 and "Clocking Off" in 2000, playing the on-screen half sister of actor Jack P. Shepherd who later became her on-screen brother David Platt in "Coronation Street". At the age of sixteen, she joined "Coronation Street" in October 1999, taking over the role of Sarah Platt from actress Lynsay King (she was three years older than her character). Sarah's pregnancy with baby Bethany Platt won O'Brien Best Storyline at the British Soap Awards. She has also won Best Newcomer at the National Television Awards and the TV Quick Awards.