| Obluchye | Obluchje |
| oboe | obo |
| Obotrites | Obotritter |
| Observable universe | Observerbart univers |
| observation | iagttagelse |
| observation satellite | observationssatellit |
| observatory | observatorium |
| Obsessive-compulsive disorder | OCD |
| Obsidian | Obsidian |
| obstacle | hindring |
| Obstetrics and gynaecology | Gynækologi |
| obstruction | hindring |
| Obzor | Horisont |
| Occam's razor | Ockhams ragekniv |
| occasion | gang |
| Occident | Vesten |
| Occitan | occitansk |
| occupation | beskæftigelse |
| Occupation of Denmark | Besættelsen |
| occupational disease | erhvervssygdom |
| occupational health | arbejdsmedicin |
| occupational health care | bedriftssundhedstjeneste |
| occupational medicine | arbejdsmedicin |
| occupational safety | arbejdssikkerhed |
| occupational safety regulation | arbejdssikkerhedsregler |
| occupational status | arbejdsstilling |
| occupied | optaget |
| occur | ske |
| occurence | tildragelse |
| ocean | ocean |
| ocean circulation | cirkulation i verdenshavene |
| ocean current | havstrøm |
| ocean dumping | dumpning i havet |
| ocean exploitation | udnyttelse af havet |
| Ocean ripple | Okeanos |
| Ocean surface wave | Bølge |
| Ocean surface waves | Bølge |
| ocean temperature | havtemperatur |
| ocean-air interface | grænseflade mellem hav og luft |
| Oceania | Oceanien |
| Oceania Football Confederation | Oceania Football Confederation |
| oceanic climate | kystklima |
| Oceanic crust | Oceanbundsplade |
| Oceanic dolphin | Delfin |
| Oceanic trench | Oceangrav |
| Oceanica | Oceanien |
| Oceanid | Okeanide |
| Oceanliner | Skib |
| oceanography | oceanografi |
| Oceanus | Okeanos |
| Ocellaris clownfish | Klovnfisk |
| Ochnaceae | Ochnaceae |
| Ocimum | Basilikum-slægten |
| Ocker | Rust |
| Oct | Oktober |
| Octal | Oktale talsystem |
| Octans | Oktanten |
| octave | oktav |
| Octave Mirbeau | Octave Mirbeau |
| Octavio Paz | Octavio Paz |
| Octet rule | Oktetreglen |
| October | oktober |
| October Revolution | Oktoberrevolutionen |
| Octobers | Oktober |
| Octonion | Oktav |
| Octopous | Ottearmet blæksprutte |
| octopus | blæksprutte |
| Octopussy | Octopussy |
| Octopussy and The Living Daylights | Octopussy and The Living Daylights |
| oculist | øjenlæge |
| Ocytocin | Oxytocin |
| OD | Od |
| Odaiba | Odaiba |
| Odd Grenland BK | ODD Grenland |
| Odd Hassel | Odd Hassel |
| Odd-toed ungulates | Uparrettåede hovdyr |
| Odda | Odda |
| Odder municipality | Odder Kommune |
| Oddset Ligaen | Superisligaen |
| Ode | Ode |
| Ode to Joy | An die Freude |
| Odense | Odense |
| Odense Boldklub | Odense Boldklub |
