O'Neill began acting in high school plays in his native Youngstown, OH. He attended college on football scholarships and went on to the Pittsburgh Steelers training camp. After a pro football career fizzled, O'Neill worked as a bellhop in Fort Lauderdale, Florida and a steel mill worker back in Youngstown. He returned to college to study acting while working as a social studies teacher. O'Neill began working regularly in local theater productions after winning the starring role in "One Flew Over the Cuckoo's Nest". He made his film debut during this period with a small role as a highway patrolman in John Boorman's "Deliverance" (1972). After making the traditional move east, O'Neill made his NYC stage debut in a off-Broadway production of Rod Serling's "Requiem for a Heavyweight." His Broadway debut followed when he replaced the fired star of "Knockout". Director William Friedkin caught the show and cast O'Neill for a small part in "Cruising" (1980).

O'Neill segued to TV as a regular on the daytime soap "Another World". He went on to guest spots, TV movies, and several busted pilots. After hitting pay dirt with "Married...", O'Neill resumed a feature career with supporting parts in "K-9" (1989), "The Adventures of Ford Fairlane" (1991), and a starring role as working-class hero "Dutch" (1991), a family comedy by John Hughes. He followed up with "Wayne's World" (1992) and its sequel "Wayne's World 2" (1993), playing a slightly unstable manager, and Friedkin's "Blue Chips" (1994), as the reporter who hounds Nick Nolte.