| Abbreviation | Definition |
|---|---|
| O | Contour Pattern |
| O | Objectives |
| O | Officer |
| O | Optional |
| O | Outstanding |
| O&I | Operations and Intelligence |
| O&O | Operational and Organizational |
| O&O | Organizations and Operations |
| O&OP | Operational and Organizational Plan |
| O&S | Operations and Support |
| O.O.D. | Operations Officer of the Day |
| O/B | Outboard |
| O/C | Observer/Collector |
| O/C | Observer/Controller |
| O1 | Officer grade: Ensign or 2nd Lieutenant |
| O10 | Officer grade: Admiral or General |
| O11 | Officer Grade: Fleet Admiral Or General of the Army |
| O2 | Object Oriented |
| O2 | Officer grade: Lieutenant Junior Grade or 1st Lieutenant |
| O2IC | Officer 2nd In Command |