O-2: Cessna Skymaster, also known as push-me-pull-you. FAC aircraft. Twin engine.
OCS: Officer's Candidate School.
OD: Olive drab color, standard "Army Green" color. Also, Officer of the Day.
ONE O DEUCE: Refers to a 105mm howitzer. Many do not know the 105mm is actually 102mm.
OPCON: Operational control.
OPEN SHEAF: A term used in calling artillery, whereby the artillery rounds were spread along an axis rather than concentrated on a single point (as when it was desired to cover a treeline).
OR: Operating room
OSCAR: Military phonetic for the letter 'O'.
OSS: Office of Strategic Services. Created in 1942, the OSS was an intelligence-gathering operation which became a forerunner of the CIA.
OUC-DA-LOI: Vietnamese for Australian.
OUT-COUNTRY: The Southeast Asian conflict outside South Vietnam (i.e., Laos and North Vietnam, sometimes Thailand, Cambodia, and China)
