| Ontariosjön | Lake Ontario |
| ontogeni | ontogenesis |
| ontologi | ontology |
| Ontologiska gudsbeviset | Ontological argument |
| OntoMeeting | OntoMeeting |
| Ontronik Khachaturian | Ontronik Khachaturian |
| ONUC | United Nations Operation in the Congo |
| onumrerad | unnumbered |
| onykter | drunk |
| onyttig | useless |
| onyttighet | uselessness |
| onyx | onyx |
| Onze-Lieve-Vrouw van het Heilig Hart | Westmalle Abbey |
| onåbar | unobtainable |
| onåd | disfavor |
| onådig | ungracious |
| onödig | dispensable |
| onödigt | unduly |
| onödigtvis | unnecessarily |
| Oogami | Oogamy |
| Oologi | Oology |
| oomtvistlig | unquestionable |
| Oomyceter | Oomycete |
| oordentlig | messy |
| oordnad | disordered |
| oordning | mess |
| Oordning -en | Combustion |
| oorganiserad | disorganized |
| oorganisk | inorganic |
| oorganisk förorening | inorganic pollutant |
| oorganisk kemi | inorganic chemistry |
| oorganiskt gödselmedel | inorganic fertiliser |
| oortodox | unorthodox |
| Oorts kometmoln | Oort cloud |
| Oostende | Ostend |
| Opabinia | Opabinia |
| opal | opal |
| opalescens | opalescence |
| opalskimrande | opalescent |
| opartisk | objective |
| opartiskhet | impartiality |
| opassande | unseemly |
| opassande beteende | indecorum |
| opassande förslag | improper suggestion |
| opasslig | indisposed |
| opastöriserad | unpasteurized |
| Opatija | Opatija |
| OPC | OLE for process control |
| OPEC | OPEC |
| Opel | Opel |
| Opel Antara | Opel Antara |
| Opel Astra | Opel Astra |
| Opel Corsa | Opel Corsa |
| Opel Frontera | Isuzu MU Wizard |
| Opel Kadett | Opel Kadett |
| Opel Meriva | Opel Meriva |
| Opel Senator | Opel Senator |
| Opel Vectra | Opel Vectra |
| Opel Zafira | Opel Zafira |
