Osterhage began his acting career in a television adaptation of True Grit in 1978 and starred in the 1979 TV movie The Legend of the Golden Gun. He is probably most recognizable to western fans in his role as Tyrel Sackett in the 1979 western The Sacketts, followed by the 1982 The Shadow Riders, both being film adaptations of novels by western Novelist Louis L'Amour. The Shadow Riders is not a part of the "Sackett" book series, and the actors play totally different roles. In both films he starred opposite Tom Selleck and Sam Elliott. In the first film he also starred alongside western legends Ben Johnson and Glenn Ford, with Johnson also starring in the second. Osterhage has appeared in twenty-seven films, made-for-television movies, and television series appearances in TV series including The Dukes of Hazzard, Knight Rider, T. J. Hooker, Scarecrow and Mrs. King, Murder, She Wrote, Moonlighting, Simon & Simon, Matlock, as well as the computer game Baldur's Gate II: Shadows of Amn. His latest appearance is in the 2008 film Taken by Force. He also played Marshal James Anderson in the computer game Outlaws, released in 1997.

Osterhage also appeared in the short-lived 1989 revival of Dragnet, called The New Dragnet, alongside Bernard White.