| Oropharyngeal airway | Canule de Guedel |
| Oropos | Oropos |
| Oroqen | Oroqen |
| Orosirian | Orosirien |
| Orosius | Paul Orose |
| Oroville | Oroville |
| Orozko | Orozko |
| orphan | orphelin |
| Orphan drug | Médicament orphelin |
| orphanage | orphelinat |
| Orphans of the Sky | Les Orphelins du ciel |
| Orphans of the Storm | Les Deux Orphelines |
| Orphean Warbler | Fauvette orphée |
| Orpheus | Orphée |
| Orpheus in the Underworld | Orphée aux Enfers |
| Orphism | Orphisme |
| Orpund | Orpund |
| Orquesta Sinfónica de Galicia | Orchestre symphonique de Galice |
| Orr | ORR |
| Orreries | Planetaire |
| Orrin Hatch | Orrin Hatch |
| Orrorin tugenensis | Orrorin tugenensis |
| Orsanmichele | Orsanmichele |
| Orsay | Orsay |
| Orseis | Orséis |
| Orselina | Orselina |
| Orsha | Orsha |
| Orsini | Famille Orsini |
| Orsini family | Famille Orsini |
| Orsières | Orsières |
| Orsk | Orsk |
| Orso I Participazio | Orso Ier Participazio |
| Orso II Participazio | Orso II Participazio |
| Orso Ipato | Orso Ipato |
| Orson Hodge | Orson Hodge |
| Orson Scott Card | Orson Scott Card |
| Orson Welles | Orson Welles |
| Ortac | Ortac |
| Ortaköy Mosque | Mosquée Ortaköy |
| Orteig Prize | Prix Orteig |
| Ortenaukreis | Ortenau |
| ORTF stereo technique | Office de radiodiffusion télévision française |
| Orthanc | Orthanc |
| Orthetrum | Orthetrum |
| Orthez | Orthez |
| Ortho-Nitrophenyl-β-galactoside | Ortho-nitrophenyl-β-galactoside |
| orthoclase | adulaire |
| orthodontics | orthodontie |
| orthodox | orthodoxe |
| Orthodox Church in Italy | Église orthodoxe en Italie |
| Orthodox Church of Constantinople | Église orthodoxe de Constantinople |
| Orthodox Judaism | Judaïsme orthodoxe |
| Orthodrome | Grand cercle |
| Orthogastropoda | Orthogastropoda |
| Orthogonal complement | Complément orthogonal |
| Orthogonal frequency-division multiplexing | Orthogonal Frequency Division Multiplexing |
| Orthogonal group | Groupe orthogonal |
| Orthogonal matrix | Matrice orthogonale |
| Orthogonal polynomials | Polynômes orthogonaux |
| orthographic | orthographique |
| orthography | orthographe |
| Orthohydrogen | Orthohydrogene |
| Orthomolecular medicine | Médecine orthomoléculaire |
| Orthomyxoviridae | Orthomyxoviridae |
| Orthonormal basis | Base de Hilbert |
| Orthonormality | Base orthonormale |
| Orthonychidae | Orthonychidae |
| orthopaedic | orthopédique |
