Born in Brazil to a British mother and businessman father, O'Neill was raised as a privileged child in such tony environments as New Rochelle, New York and Wilton, Connecticut. As a teenager, the leggy brunette won several awards for her horsemanship. By age 15, the beauty had been put under contract by the Ford modeling agency and soon was appearing on magazine covers and in TV commercials. Like many other models, O'Neill gravitated to acting, making her film debut in a bit role in "For the Love of Ivy" (1968) and acquitted herself opposite John Wayne in Howard Hawks' final film "Rio Lobo" (1970). After her breakthrough in "Summer of '42", the actress was in demand but was merely window dressing in Otto Preminger's soapy "Such Good Friends" (1972). O'Neill delivered, however, as a woman whose lover could possibly be harboring the spirit of her dead father in the underrated "The Reincarnation of Peter Proud" (1975). Her subsequent feature output, however, has been of mixed quality, with the best being David Croenenberg's "Scanners" (1991) and "Committed" (1993), in which she was a nurse-turned-patient in a mental institution.
O'Neill fared slightly better on the small screen. She was quite good as a Southerner brutalized in a Union prison during the Civil War in "Love's Savage Fury" (ABC, 1979) and was properly haughty as the titled, globe-trotting mother of a perfume heiress in "Bare Essence" (NBC, 1983). The actress had her best chance as a fashion photographer who was really a government agent in "Cover Up" (CBS, 1984-85), but the untimely death of her co-star Jon-Eric Hexum cast a pall over the project. Throughout the late 80s and into the 90s, O'Neill has kept busy in TV-movies, playing everything from a psychotic murderer in "Red Spider" (CBS, 1988) to a journalist who becomes involved with an inmate in "Invasion of Privacy" (USA Network, 1992) to Richard Crenna's wife in "Jonathan Stone: Threat of Innocence" (NBC, 1994).