oath of allegiance (to the colors)
objective rally(ing) point (ORP)
objective rally point / objective rallying point
oblique fire; fire to the oblique
obscurant agent (vision-restricting agent)
observation aircraft (O; obsn acft)
observation and fields of fire
observe, orient, decide, act (OODA loop)
obstacle clearance altitude (OCL)
obstacle clearance limit (OCL)
obstacle intelligence (OBSTINTEL)
off-gauge cargo; out-/ oversized cargo
off-route mine (ORM); horizontal action mine; horizontal effects mine; side-hitting mine
offensive electronic warfare operation
office of civil and defense mobilization
Office of Homeland Security (OHS)
Office of NATO Standardization
Office of NATO Terminology Coordination
Office of Naval Intelligence (ONI)
Office of the Assistant Chief of Staff for Installation Management (OACSIM)
office of the chief of special warfare
Office of the Secretary of Defense (OSD)
Office of the Secretary of the Navy
Office of the Special Representative of the Secretary-General for West Africa (UNOWA)
office of the under secretary of defense for acquisition and technology
Office of the United Nations Special Coordinator in the Occupied Territories (UNSCO)
Office of U.S. Foreign Disaster Assistance (OFDA)
officer commissioned from the ranks
officer conducting the exercise (OCE)
officer in charge of logistics
officer leadership training course
officer of primary responsibility