| Operation Guds Vrede | Operation Wrath of God |
| Operation helvede | They Were Expendable |
| Operation Overlord | Battle of Normandy |
| Operation Panzerfaust | Operation Panzerfaust |
| Operation Restore Hope | Operation Restore Hope |
| Operation Torch | Operation Torch |
| Operation Vigilant Resolve | First Battle of Fallujah |
| Operation Weserübung | Operation Weserübung |
| operationskniv | scalpel |
| Operationsstue | Operating theatre suite |
| operativsystem | operating system |
| Operatorprioritet | Order of operations |
| operette | operetta |
| Opfattelse | Thought |
| Opfindelse | Invention |
| opfostring | upbringing |
| opførelse | building |
| opførsel | demeanor |
| opgave | assignment |
| opgive | abandon |
| opgravet mudder | dredged material |
| opgørelse over skovskader | inventory of forest damage |
| Ophavsret | Copyright |
| ophedning | heating |
| Ophelia | Ophelia |
| Ophidiofobi | Ophidiophobia |
| ophidset | excited |
| ophold | sojourn |
| ophøre | cease |
| Opildner | Rulling spirit |
| opinion | opinion |
| opinionsundersøgelse | opinion survey |
| opium | opium |
| Opiumskrigene | Opium Wars |
| Opkast | Vomiting |
| Oplader | Cartridge |
| oplagt | transparent |
| oplevelse | adventure |
| oplyse | light |
| oplysningspligt | obligation to inform |
| oplysningstiden | Age of Enlightenment |
| opløse | disintegrate |
| opløselighed | solubility |
| opløsning | resolution |
| opløsningsmiddel | solvent |
| opløst ilt | dissolved oxygen |
| opløst organisk kulstof | dissolved organic carbon |
| Opmærkningssprog | Markup language |
| opmærksom | attentive |
| opnå | attain |
| Opole | Opole |
| Oppdal | Oppdal |
| Oppegård | Oppegård |
| Oppidum | Oppidum |
| Oppland | Oppland |
| Opportunity | Opportunity rover |
| oppvaskemaskine | dishwasher |
| Oprah Winfrey | Oprah Winfrey |
| oprensningsfartøj til oliespild | oil recovery vessel |
| Opret Kobjælde | Pulsatilla vulgaris |
| Oprette | Institute |
| Oprindelige amerikanere | Indigenous peoples of the Americas |
| oprør | revolt |
| oprørt | upset |
| opsamle | accrue |
| opsamling | catchment |
| Opsamlingsalbum | Compilation album |
| opsamlingsheat | repechage |
| opsige | denounce |
| opskrift | recipe |
| opspare | save |
| opspind | fiction |
| opsporing | detection |
| opstand | revolt |
| Opstandelse | Resurrection |
| Opstigende knudes længde | Longitude of the ascending node |
| opstigning | ascent |
| Optegnelser fra et kælderdyb | Notes from Underground |
| Opteron | Opteron |
| optik | optics |
| Optimater | Optimates |
| Optimering | Optimization |
| Optimus-tastatur | Optimus keyboard |
| Optisk linse | Lens |
| Optisk prisme | Prism |
| optog | train |
| optræden | demeanor |
| Opus | Opus number |
| Opus Dei | Opus Dei |
| Oracle | Oracle Corporation |
| Oradea | Oradea |
| Oradour-sur-Glane | Oradour-sur-Glane |
| Orakelben | Oracle bone |
| Oraklet i Delfi | Delphic Sibyl |
| oralsex | oral sex |
| orange | orange |
| Orange høsommerfugl | Anxiety to |
| Orange Range | Orange Range |
| Orange River Colony | Orange River Colony |
| Orangutang | Orangutan |
| Oranien | House of Orange-Nassau |
| Oranienburg | Oranienburg |
| Oranje | Orange |
| Oratorium | Oratorio |
| Orbiter Space Flight Simulator | Orbit |
| ord | word |
| ord for ord | word for word |
| ord til andet | word for word |
| ordblindhed | dyslexia |
| ordbog | dictionary |
| ordbogsangreb | dictionary attack |
| ordbøger | dictionary |
| Orden | Order |
