| Abbreviation | Definition |
|---|---|
| O5 | Overseer Level 5 |
| OAA | One Against All |
| OASE | Other Age Second Encounter |
| OBBY | Obsidian |
| OBBY | Obstacle By |
| obby | Obstacle course |
| OBLV | Oblivion PVM (gaming) |
| OBND | Openbve News and Development (gaming forum) |
| OBT | Open Beta Testing |
| OCBG | Orange County Board Gamers |
| OCG | Office of Charitable Gaming |
| OCGA | Ontario Charitable Gaming Association |
| OCW | Original Create a Wrestler |
| ODM | Oderint Dum Metuant |
| OELVN | Original English-Language Visual Novel |
| OG | Odyssey Gaming |
| OG | Original Gamer |
| OGA | Online Gaming Addicts |
| OGA | Online Gaming Alliance |
| OGA | Open Gaming Alliance |