She portrayed the Captain Rachel Garrett of the U.S.S. Enterprise (NCC-1701-C) in Star Trek: The Next Generation in the episode "Yesterday's Enterprise" and the Klingon Kurak, in the episode "Suspicions". Later she guest-starred on Star Trek: Deep Space Nine as Cardassian Korinas in the episode "Defiant".
Other appearances include as nightclub singer Julie Heller in the episode "Murder! Murder!" of The Eddie Capra Mysteries (1978, NBC), as Dorothy Fulton in Hart to Hart (1979, ABC), as female stunt woman "Charlie" in the episode of the same name in the first season of The Fall Guy (1981, ABC), in Remington Steele (1982), as conniving "other woman" Ashley Vickers in the pilot episode of Murder, She Wrote (1984, CBS), as a pushy reporter in the second-season episode "Catch Of The Day" in Riptide (1984, NBC), as the owner of a travelling wild west rodeo show in the third-season Airwolf episode "Annie Oakley" (1985), and separate roles in three episodes of Matlock from 1989-1994.
She also guest-starred in the Babylon 5 season-one episode "Believers" (1994) as "M'ola". Later on, she played the Earth Alliance President in the Babylon 5 TV movie Babylon 5: In the Beginning (1998). O'Neil's film credits include roles in The Legend of Nigger Charley (1972), The Gumball Rally (1976), Mary Jane Harper Cried Last Night (1977), Are You in the House Alone? (1978), The Kid from Left Field (1979), Brave New World (1980), Piranha II: The Spawning (1981), Jacqueline Susann's Valley of the Dolls (1981), Ted & Venus (1991) and Titanic (1997). She received a Theatre World Award for her performance in the Broadway musical Two by Two with Danny Kaye.
