Having graduated from school, she was tutored in drama classes under Dorothy Hemingway and started her career as a stage performer in the 1950s with the Mosman Theatre Company.
[4] She trained in radio under Rosalind Kennerdale, and through her gained agent John Cover, the husband of actress Queenie Ashton, who had a company, Central Casting.
O'Sullivan appeared in numerous radio productions, such as Dr. Paul, When a Girl Marries, Blue Hills, In a Sunburnt Country and Life with Dexter.
O'Sullivan never had any other long-standing roles; she was a well-recognized feature player as a prominent guest artist of many a television series spanning over 60 years, later portraying hapless old grannies.
[5] O'Sullivan also became popular via memorable guesting appearances in Cop Shop as Lorna Close (later O'Reilly), mother to Valerie Close-Johnson and mother-in-law to Jeffrey Johnson,[6] and in Hey Dad..!