| Acronym | Definition |
|---|
| O | Oh |
| O | Outstanding |
| O | Oxygen |
| O | Officer (military) |
| O | Operator (telephone) |
| O | Octal |
| O | Official |
| O | Organization |
| O | Outdoor |
| O | Orange |
| O | Out (baseball) |
| O | Operations |
| O | New Orleans, Louisiana (mint mark) |
| O | Oscar (aviation letter code) |
| O | Output |
| O | Offensive (Catholic movie rating) |
| O | Orgasm |
| O | Ortho (Benzene) |
| O | Hug(s) |
| O | Operational |
| O | Observer |
| O | Organizational |
| O | Order of Magnitude (computer algorithms) |
| O | Oxide |
| O | Otis (Redding) |
| O | Immediate (logging abbreviation) |
| O | Odin |
| O | Ohm |
| O | Oscar Robertson (former NBA player) |
| O | Oprah (magazine) |
| O | Ouest (French: west) |
| O | Othello |
| O | Observation (US military aircraft designation; as in O-1) |
| O | Orthogonal (mathematics) |
| O | Obstacle (course) |
| O | Defensive Team (sports/coaching) |
| O | Omit |
| O | Outliers |
| O | Omikron (German: Omicron; 15th letter of the Greek alphabet) |
| O | Oberst (German) |
| O | Over to You (radio communications) |
| O | Morally Offensive (Catholic movie rating) |
| O | Origin Server (Internet) |
| O | Organized Naval Reserve (US Navy) |
| O | Optional Levels (board test for high schoolers, UK) |
| O | Officer of Operations (US Navy) |
| O | Ohne Hauch (antigen) |
| O | Ost/Osten (German: East) |
| O | Measurement of Cloud Cover |
| O | Toy train railroad gauge (2 1/4 inch wide, originated in Germany) |