O
- OBR – Optical Beam-Riding
- OCC – Obus à Charge Creusé (shaped-charge shell)
- OCOKA – Observation and fields of fire, Cover and concealment, Obstacles, Key terrain, and Avenues of Approach.
- OCSW – Objective Crew-Served Weapon
- OCU – Operational Centre Unit
- ODE – Ordnance Development and Engineering (Singapore)
- OEG – Occluded Eye Gunsight (a type of Collimator sight)
- OEO – optical and electro-optical
- OFSA – Objective Family of Small Arms
- OFSA – One Size Fits All
- OFW – Objective Force Warrior
- OHWS – Offensive Hand Weapon System
- OIC – Officer In Charge
- OICW – Objective Individual Combat Weapon
- OOTW – Operations Other Than War
- OP – Observation post
- OP – Operators Panel
- OPCON – OPerational CONtrol
- OPDW – Objective Personal Defense Weapon
- OPORD – OPerations Order
- OPSEC – OPerational SECurity
- OPV – Observation Post Vehicle
- OSW – Objective Sniper Weapon
- OSS (USAF) – Operational Support Squadron
- OSW (USAF) – Operational Support, Weather
- OT – operational test
- OTA – overflight top attack
- OTEA – Operational Test and Evaluation Agency (US)
Read more about this topic: Glossary Of Military Abbreviations