| Abbreviation | Definition |
|---|---|
| OIA | Organization of Istanbul Armenians of Los Angeles |
| OIC | Movimiento Comunista de las Islas - Organización de la Izquierda Comunista |
| OIC | Organisation of Islamic Countries |
| OIC | Organización de Izquierdistas Comunistas de España |
| OIC | Organization of the Islamic Conference |
| OIFY | Once in five years (a once-in-five-years political transition) |
| OIRA | Official Irish Republican Army |
| OK | Old Kinderhook |
| OKDE | Organosi Kommouniston Diethniston Ellados (Greek: Organization of the Communist Internationalists of Greece) |
| OLO | Office of the Leader of the Opposition |
| OLP | Ontario Liberal Party |
| OMT | Organizacao de Mulher Timor |
| ONAC | Oklahoma Native Assets Coalition |
| ONAC | Oregon Native American Chamber |
| ONDP | Ontario New Democratic Party |
| ONDY | Ontario New Democratic Youth |
| ONP | Onafhankelijke Nijmeegse Partij |
| OOPS | Office Of Political Suicide |
| OOTD | Orders Of The Day |
| OPAC | Oil Producing Arab Countries |