Her acting career began in her adolescence, when she was chosen to play the part of Carrie Rawlins in the Disney feature film Bedknobs and Broomsticks in 1971.
She has appeared in a wide range of television programmes and films since, including BBC's EastEnders, where she played Andrea Price for several stints in the 1990s.
O'Callaghan played Carrie Rawlins in Bedknobs and Broomsticks (1971), where she starred opposite Angela Lansbury and David Tomlinson.
She has appeared in numerous television programmes throughout the 1970s, '80s, '90s and early 2000s, including ITV's The Bill, Casualty, Specials, Boon, Rumpole of the Bailey, Woof!
More recently she is known for her role as Andrea Price—the "boozy" mother of Natalie Evans (Lucy Speed)—in the BBC soap opera EastEnders (1994–1995; 1999).