Corporal Walter "Radar" O'Reilly was played by actor
Gary Burghoff, who was the only actor to star in both the
M*A*S*H movie and TV series.
Radar got his nickname from being able to hear incoming helicopters before anyone else. He also had a tendency to know what Lt. Colonel Henry Blake (and later, Colonel Sherman Potter) were going to say before they even said it. A usual scene would be Radar already repeating and acknowledging order the instant before the commanding officer would give them. His height was also a source of many jokes on the show.
It was noted in several episodes that Corporal O'Reilly was from Ottumwa, Iowa.