- Origin of Species, The
- origin story
- orihon
- orihou
- Orillia
- orinasal
- Orinda
- O-ring
- Orinoco
- oriole
- Orion
- Orionids
- Orion Nebula
- orisha
- Oriskany
- orismology
- orison
- Orissa
- orixa
- Oriya
- Orizaba
- Orjonikidze
- Orkhon
- Orkney
- Orkney Islands
- Orlando
- Orland Park
- orle
- Orléanais
- Orleanist
- Orléans
- Orléans, d'
- orlfly
- Orlich
- orlo
- Orlon
- orlop
- Orlov
- Orly
- Ormandy
- Ormazd
- ormer
- ormolu
- Ormond Beach
- Ormonde
- Ormuz
- ornament
- ornamental
- ornamentalism
- ornamentation
- ornamented
- ornate
- ornately
- Orne
- ornery
- ornis
- ornith-
- ornith.
- ornithic
- ornithine
- ornithischian
- ornitho-
- ornithoid
- ornithol.
- ornithological
- ornithologist
- ornithology
- ornithomancy
- ornithophilous
- ornithophily
- ornithopod
- ornithopter
- ornithorhynchus
- ornithoscopy
- ornithosis
- oro
- oro-
- orobanchaceous
- orogen
- orogeny
- orographic cloud
- orographic precipitation
- orography
- oroide
- orometer
- Oromo
- oronasal
- Orono
- Orontes
- oropendola
- oropharynx
- Orosius
- or other
- orotund
- oro y plata
- Orozco
- Orpen
- orphan
- orphanage
- orphan drug
- orphans' court
- orpharion
- Orphean
- Orpheus
- Orpheus and Eurydice
- Orphic
- Orphism
- orphrey
- orpiment
- orpine
- Orpington
- Orr
- orra
- orrery
- Orrin
- orris
- orrisroot
- Orsha
- Orsini
- Orsk
- or so
- Orson
- ort
- ortanique
- Ortega
- Ortegal
- Ortega Saavedra
- Ortega y Gasset
- orth-
- orth.
- Orth.
- orthicon
- ortho
- ortho-
- orthoboric acid
- orthocenter
- orthocentre
- orthocephalic
- orthochromatic
- orthoclase
- orthoclastic
- ortho-cousin
- ortho-dichlorobenzene
- orthodontia
- orthodontics
- orthodontist
- orthodonture
- orthodox
- Orthodox
- Orthodox Christianity
- Orthodox Church
- Orthodox Jew
- Orthodox Judaism
- orthodox sleep
- orthodoxy
- orthoepy
- orthoformate
- orthoformic acid
- orthogenesis
- orthogenetic
- orthogenic
- orthognathic surgery
- orthognathous
- orthogonal
- orthogonalize
- orthogonal matrix
- orthogonal projection
- orthogonal trajectory
- orthographer
- orthographic
- orthographic projection
- orthographize
- orthography
- orthohydrogen
- orthokeratology
- orthomolecular
- orthomorphic
- orthomorphic projection
- orthonormal
- Ortho-Novum
- orthopaedic
- orthopaedics
- orthopedic
- orthopedics
- orthopedist
- orthophosphate
- orthophosphoric acid
- orthophosphorous acid
- orthophthalic acid
- orthophyric
- orthopnea
- orthopod
- orthopraxia
- orthopraxy
- orthopsychiatry
- orthopter
- orthopteran
- orthopteron
- orthopterous
- orthoptic
- orthoptics
- orthopyroxene
- orthorexia
- orthorhombic
- orthoscope
- orthoscopic
- orthoselection
- orthosis
- orthostat
- orthostates
- orthostatic
- orthostatic hypotension
- orthostichy
- orthostyle
- orthotic
- orthotics
- orthotist
- ortho-toluidine
- orthotone
- orthotropic
- orthotropism
- orthotropous
- ortho-xylene
- orthros
- Ortler
- Ortles
- ortolan
- Orton
- orts
- Orumiyeh
- Oruro
- ORV
- Orvieto
- Orville
- Orwell
- Orwell, George
- Orwellian
- or what
- or whatever
- Oryol
- oryx
- orzo
- orˈthoptist
- os
- Os
- OS
- OSA
- Osage
- Osage orange
- Osaka
- Osasco
- OSB
- Osbert
- Osborn
- Osborne
- Osborne House
- Oscan
- oscar
- Oscar
- Oscar II
- OSCE
- Osceola
- oscillate
- oscillating circuit
- oscillating engine
- oscillating universe
- oscillating universe theory
- oscillation
- oscillator
- oscillatory
- oscillogram
- oscillograph
- oscillometer
- oscilloscope
- oscine
- oscitancy
- oscitant
- Osco-Umbrian
- osculant
- oscular
- osculate
- osculating circle
- osculating plane
- osculation
- osculum
- OSD
- Osee
- Ösel
- Osetic
- OSF
- OSFCW
- Osgood
- Osh
- OSHA
- Oshawa
- Oshkosh
- Oshogbo
- Oshun
- OSI
- osier
