In 1967, the Air Force needed a twin engine forward observation aircraft for Vietnam. Cessna proposed its civilian Model 337 which was currently in production. Modifications for military use included pylons for smoke rockets, a smoke generator to make it more visible, and a loud speaker for psychological warfare.
The Air Force first used the Skymaster in Da Nang in July 1967. Many still are used by the Air National Guard. The O-2A at the Castle Air Museum came from the California Air National Guard at Van Nuys.