| Abbreviation | Definition |
|---|---|
| OIE | Office of International Education |
| OIE | World Organisation for Animal Health |
| OIH | Office of International Health |
| OIOS | ffice of Internal Oversight Services |
| OIOS | Office of Internal Oversight Services |
| OLA | ffice of Legal Affairs |
| OLA | Office of Legal Affairs |
| OMUN | Ohio Model United Nations |
| OMUN | Oklahoma Model United Nations |
| OMUN | Online Model United Nations |
| OMUN | Ontario Model United Nations |
| ONU | Organización de las Naciones Unidas |
| ONU | Organizzazione delle Nazioni Unite |
| ONUCI | Opération des Nations Unies en Côte d'Ivoire |
| ONUCI | United Nations Operation in Côte d'Ivoire |
| OO | Official Observer |
| OP | Observation Point |
| OPCAT | Optional Protocol to the UN Convention against Torture |
| OPCW | Organisation for the Prohibition of Chemical Weapons |
| OPM | Other People's Money |