What was the first name of Radar O'Reilly?

The first name of Radar O’Reilly was Walter. On the television series M*A*S*H, he was generally identified as Corporal Walter Eugene “Radar” O’Reilly, a young company clerk whose extraordinary intuition earned him the nickname “Radar.”

Radar was portrayed by Gary Burghoff, who played the character in both the 1970 film and the television series. He was the only major actor to appear in both versions. The television character came from Ottumwa, Iowa, and joined the Army directly after high school, becoming known for anticipating messages and appearing beside commanding officers before they called for him.

There is a small continuity detail behind this answer: Radar’s first name was not established in the original novel or film in the same way. The television series made “Walter” part of his identity, and Burghoff reportedly helped choose it. The later spin-off pilot W*A*L*T*E*R also used the name.