O'Neil made her film debut in the 1972 filmThe Legend of Nigger Charley. Other film appearances include The Gumball Rally, Mary Jane Harper Cried Last Night, Are You in the House Alone?, The Kid from Left Field, Brave New World, ', Ted & Venus and Titanic. O'Neil made her television debut in the 1973 television filmDuty Bound. She appeared in a number of one-off appearances in guest roles on various television series, including a dog trainer in a 1978 episode of Columbo titled "How to Dial a Murder", nightclub singer Julie Heller in the episode "Murder! Murder!" of The Eddie Capra Mysteries, as Dorothy Fulton in Hart to Hart, as female stunt woman "Charlie" in the episode of the same name in the first season of The Fall Guy, in Remington Steele, as conniving "other woman" Ashley Vickers in the pilot episode of Murder, She Wrote, as a pushy reporter in the second-season episode "Catch Of The Day" in Riptide, as the owner of a travelling wild west rodeo show in the third-season Airwolf episode "Annie Oakley", and separate roles in three episodes of Matlock from 1989-1994. O'Neil appeared in the 6-hour television miniseriesJacqueline Susann's Valley of the Dolls, and appeared in two episodes of The A-Team, playing Dr. Maggie "Mo" Sullivan in the season one episode "Black Day at Bad Rock" and the season two episode "Deadly Maneuvers". O'Neil made a number of appearances in popular science fiction television series during the 1980s and 90s. She portrayed Captain Rachel Garrett of the U.S.S. Enterprise in the ' episode "Yesterday's Enterprise", returning to that series with a role as the Klingon Kurak, in the episode "Suspicions |Suspicions". Later she guest-starred on ' as Cardassian Korinas in the episode "Defiant |Defiant". O'Neil guest-starred in the Babylon 5 season-one episode "Believers" as "M'ola". Later on, she played the Earth Alliance President in the Babylon 5 TV movie '. In 1991, O'Neil's filmed her scenes in the role of Hoelun for the never-released film Genghis Khan. Efforts in 2010 to repackage the material as a miniseries, tentatively named Genghis Khan: The Story of a Lifetime, never came to fruition. O'Neil's last screen appearance was on the television series JAG, playing Dr. Beth Salluci in the 2001 episode "Redemption".