In the cinema, O'Mara acted in two 1970 Hammer Horror films: "The Vampire Lovers" and "The Horror of Frankenstein". On BBC television, she had regular roles in "The Brothers" (1975–1976), "Triangle" (1981–1982) and "Howards' Way" (1989–1990), and portrayed "Doctor Who" villain the Rani three times (1985–1993). She also appeared as Jackie Stone in two episodes of the sitcom "Absolutely Fabulous" (1995–2003). On American television, she played Caress Morell, the scheming sister of Alexis Colby in the primetime soap opera "Dynasty" (1986).