| Abbreviation | Definition |
|---|---|
| ORTC | Operational Readiness Training Complex |
| ORTSU | Organized Reserve Training Service Training Unit |
| ORTSU | Organized Reserve Training Service Unit |
| ORX | Operational Receiver |
| OS | Old Skool |
| OS | On Sea |
| OS | On Service |
| OS | On Station |
| OS | Operating Strength |
| OS | Operating System |
| OS | Operations Specialist |
| OS | Opportunity to Serve |
| OS | Ordinary Seaman |
| OS | Ordnance Survey |
| OS | Other Specifications |
| OS | Outer Sensor |
| OS | Overtly Similar |
| OSA | Office of Special Affairs |
| OSA | Office of the Secretary of the Army |
| OSA | Open System Architecture |