She started her acting career in 1974 as a cast member of The Second City in Toronto. In 1976, this theatre troupe created the television series SCTV, on which O'Hara became a regular performer. In the late 1970s, she did various voice-overs for a number of cartoons, which would continue throughout her career.
She has also appeared in world-reknown Canadian comedy, SCTV. She's appeared several times with fellow Canadian John Candy.