He was active in American films and television, such as his memorable supporting role as the trial judge in the Sidney Lumet-directed movie The Verdict (1982) with Paul Newman, an episode of The Golden Girls in 1987, and portraying Chief Justice of the United States Roy Ashland in the television series The West Wing. In 1992, O'Shea guest starred in the season 10 finale of the sitcom Cheers, and, in 1995, in an episode of the show's spin-off Frasier. In the episode of Frasier, he played Dr. Schachter, a couples therapist who counsels the Crane brothers together. He appeared in the pilot episode of Early Edition as Sherman.