| Orthodoxe Kerk in de Tsjechische Landen en Slowakije | Czech and Slovak Orthodox Church |
| Orthodoxe kerken | Eastern Orthodox Church |
| Orthogonal frequency division modulation | Orthogonal frequency-division multiplexing |
| orthografie | orthography |
| orthografisch | orthographic |
| Orthografische cilinderprojectie | Lambert cylindrical equal-area projection |
| Orthomoleculaire therapie | Orthomolecular medicine |
| Orthonectida | Orthonectida |
| Orthopantomogram | Orthopantomogram |
| orthopedie | orthopedics |
| orthopedisch | orthopedic |
| orthopedist | orthopedist |
| orthopediste | orthopedist |
| orthopeed | orthopedist |
| Orthoptera | Orthoptera |
| Orthorexia nervosa | Orthorexia nervosa |
| Orthosie | Orthosie |
| orthosympatisch zenuwstelsel | sympathetic nervous system |
| Orthotroop | Orthotropic material |
| Ortler | Ortler |
| Orto botanico di Padova | Orto botanico di Padova |
| Orto botanico di Pisa | Orto botanico di Pisa |
| Ortrand | Ortrand |
| Ortygia | Ortygia |
| Orujo | Orujo |
| Oruro | Oruro Department |
| Orust | Orust Municipality |
| Orval | Orval Brewery |
| Orvieto | Orvieto |
| Orvin | Orvin |
| Orwelliaans | Orwellian |
| Oryctodromeus | Oryctodromeus |
| Orzens | Orzens |
| Orzesze | Orzesze |
| Orzełklasse | Orzeł class submarine |
| Orzinuovi | Orzinuovi |
| Orzysz | Orzysz |
| os | steer |
| Os Blancos | Os Blancos |
| Os cuneiforme intermedium | Intermediate cuneiform bone |
| Os cuneiforme laterale | Lateral cuneiform bone |
| Os cuneiforme mediale | Medial cuneiform bone |
| Os naviculare | Navicular bone |
| Osadné | Osadné |
| Osage County | Osage County |
| Osagedoorn | Osage-orange |
| Osaka | Osaka |
| Osaka Aquarium Kaiyukan | Osaka Aquarium Kaiyukan |
| Osaka-ringlijn | Osaka Loop Line |
| Osam | Osam |
| Osama bin Laden | Osama bin Laden |
| Osama bin Laden mogelijk overleden | French newspaper suggests Osama bin Laden may be dead |
| Osama Moustafa Hassan Nasr | Hassan Mustafa Osama Nasr |
| Osamu Tezuka | Osamu Tezuka |
| Osann-Monzel | Osann-Monzel |
| Osbourne Reynolds | Osborne Reynolds |
| Osburg | Osburg |
| Osby | Osby Municipality |
| Oscar | Oscar |
| Oscar Bernadotte | Count Oscar Bernadotte of Wisborg |
| Oscar Camenzind | Oscar Camenzind |
| Oscar de la Hoya | Oscar de la Hoya |
| Oscar Galíndez | Oscar Galíndez |
| Oscar Gutierrez | Oscar Gutierrez |
| Oscar Hammerstein II | Oscar Hammerstein II |
| Oscar het Moppermonster | Oscar the Grouch |
| Oscar I van Zweden | Oscar I of Sweden |
| Oscar II van Zweden | Oscar II of Sweden |
| Oscar Mathisen-trofee | Oscar Mathisen Award |
| Oscar Montelius | Oscar Montelius |
| Oscar Niemeyer | Oscar Niemeyer |
| Oscar Peterson | Oscar Peterson |
| Oscar Ruiz | Óscar Ruiz |
| Oscar van Dillen | Oscar van Dillen |
| Oscar Wilde | Oscar Wilde |
| Oschatz | Oschatz |
| Oschersleben | Oschersleben |
| oscillatie | oscillation |
| oscillator | oscillator |
| oscilleren | oscillate |
| oscillerend | oscillatory |
| oscillograaf | oscilloscope |
| Osco | Osco |
| osculatie | osculation |
| OSDL | Open Source Development Labs |
| Oseltamivir | Oseltamivir |
| Osen | Osen |
| Osgiliath | Osgiliath |
| Oshikoto | Oshikoto Region |
| Oshiwambo | Ndonga language |
| OSI-model | OSI model |
| Osieczna | Osieczna |
