Corporal “Radar” O’Reilly is a fictional character in the M*A*S*H novels, the film, the television series, the television movie, W*A*L*T*E*R, and two episodes of the series, After MASH. The character was portrayed by Gary Burghoff in both the film and on television — the only actor from the film to reprise his role on television, aside from G. Wood as General Hammond.

In the Richard Hooker novels, Radar's full name is given as J. Robespierre O'Reilly, while on the TV series it is Walter Eugene O'Reilly. (He is simply "Radar O'Reilly" in the film.)