- OAO Otkrytoye Aktsionernoye Obshchestvo (JSC; Russian company constitution).
- OAT Outside air temperature.
- OBIGGS Onboard inert gas generating system.
- OBOGS Onboard oxygen generating system.
- OCU (1) Operational Conversion Unit; (2) operational capabilities upgrade.
- OEI One engine inoperative.
- OEU Operational Evaluation Unit.
- offset Workshare granted to a customer nation to offset the cost of an imported system.
- OGE Out of ground effect; helicopter hovering, far above nearest surface.
- OKB Opytnyi Konstruktorskoye Byuro (Russian experimental design bureau).
- Omega Long-range hyperbolic radio navaid.
- OOO Obshchestvo Ogranichennoye Otvetstvennostyu (Russian company constitution).
- opeval Operational evaluation.
- OTH Over-the-horizon (OTHT adds targeting).
- OTPI On-top position indicator (indicates overhead of submarine in ASW).
- OWE Operating weight empty. MTOW minus payload, usable fuel and oil and other consumables (thus, includes crew).
