| ontano nero | black alder |
| Ontario | Ontario |
| Ontelaunee Township | Ontelaunee Township |
| Ontinyent | Ontinyent |
| ontogenesi | ontogenesis |
| ontologia | ontology |
| ontologico | ontological |
| OntoMeeting | OntoMeeting |
| Ontoso | Shame-faced |
| ONU | UN |
| ONUB | United Nations Operation in Burundi |
| ONUC | United Nations Operation in the Congo |
| ONUMOZ | United Nations Operation in Mozambique |
| Onuris | Anhur |
| Onēsama | Oneesama |
| OO | OO |
| Oocito | Ovum |
| Oodaaq | Oodaaq |
| Ooliti | Roestone |
| Oomycota | Oomycete |
| Ooogamia | Ooogamy |
| OOPArt | Out-of-place artifact |
| Oorlans | Oorlans |
| Oost-Vlaams | East Flemish |
| Ooteca | Ootheca |
| OP | OP |
| OPA | OPA |
| OPAC | OPAC |
| opacità | impenetrability |
| opaco | cloudy |
| opale | opal |
| opalescente | opalescent |
| opalescenza | opalescence |
| Opatovce | Opatovce |
| Opatovce nad Nitrou | Opatovce nad Nitrou |
| Opava | Opava |
| OPEC | OPEC |
