Oppenheimer is recognized as the voice of many characters, often for Filmation in the 1970s and 1980s, such as Mighty Mouse, Ming the Merciless on Flash Gordon, the Overlord on BlackStar, Skeletor, Man-At-Arms and Mer-Man from Filmation's 1980's cartoon He-Man and the Masters of the Universe, and the voice of Prime Evil in the 1986 TV series, Filmation's Ghostbusters. Other notable voice roles include Thundarr the Barbarian, Vanity on The Smurfs, Rhinokey and Crock from The Wuzzles and Falkor, Gmork, Rockbiter, and the Narrator from 1984's The NeverEnding Story. In the early 1990s, Oppenheimer was the voice of Merlin in The Legend of Prince Valiant. He also provided the voice of Barkerville in the Pound Puppies TV special. He also voiced Fraidy Cat on Fraidy Cat in 1975.