| Abbreviation | Definition |
|---|---|
| O2O | Observe to Obey |
| O3 | Officer grade: Lieutenant or Captain |
| O4 | Officer grade: Captain or Colonel |
| O4 | Officer grade: Commander or Lieutenant Colonel |
| O4 | Officer grade: Lieutenant Commander or Major |
| O5 | Officer grade: Commander or Lieutenant Colonel |
| O5 | Österreich (Austrian) Resistance Group |
| O6 | Officer grade: Captain or Colonel |
| O7 | Officer grade: Commodore, Rear Admiral (Lower Half), or Brigadier General |
| O8 | Officer grade: Rear Admiral (Upper Half), or Major General |
| O9 | Officer grade: Vice Admiral or Lieutenant General |
| OA | Objective Area |
| OA | Operating Assembly |
| OA | Operational Area |
| OA | Operational Assessment (or Architecture) |
| OA | Operations Aerology Shipboard METOC Division |
| OA | Over Aged |
| OA | Overall |
| OAB | Oakland Army Base |
| OAC | Optical Area Correlator |