- oca
- ocarina
- OCasey
- Occam
- Occams Razor
- occasion
- occasional
- occasionally
- occasions
- Occident
- Occidental
- occidentalise
- Occidentalism
- occidentalize
- occipital
- occipital bone
- occipital cortex
- occipital gyrus
- occipital lobe
- occipital protuberance
- occipital vein
- occipitomastoid suture
- occiput
- Occitan
- occlude
- occluded
- occluded front
- occlusion
- occlusive
- occult
- occult arts
- occultation
- occultism
- occultist
- occupancy
- occupancy rate
- occupant
- occupation
- occupation licence
- occupation license
- occupational
- occupational disease
- occupational group
- occupational hazard
- occupational safety and health act
- Occupational Safety and Health Administration
- occupational therapy
- occupied
- occupier
- occupy
- occur
- occurrence
- occurrent
- ocean
- ocean bottom
- ocean current
- ocean floor
- ocean liner
- ocean perch
- ocean pout
- Ocean State
- ocean sunfish
- ocean trip
- oceanaut
- oceanfront
- oceangoing
- Oceania
- Oceanic
- oceanic abyss
- oceanic bird
- oceanic bonito
- oceanic whitetip shark
- Oceanica
- Oceanid
- Oceanites
- Oceanites oceanicus
- oceanographer
- oceanography
- oceanology
- Oceanus
- ocellated
- ocellated turkey
- ocellus
- ocelot
- ocher
- ochlocracy
- Ochna
- ochna family
- Ochna serrulata
- Ochnaceae
- Ochoa
- Ochotona
- Ochotona collaris
- Ochotona princeps
- Ochotonidae
- ochre
- Ochroma
- Ochroma lagopus
- ochronosis
- Ochs
- Ocimum
- Ocimum basilicum
- Ockham
- Ockhams Razor
- oclock
- oconee bells
- OConnor
- ocotillo
- Oct
- octad
- octadecanoic acid
- octagon
- octagonal
- octahedron
- octal
- octal digit
- octal notation
- octal number system
- octal numeration system
- octameter
- octane
- octane number
- octane rating
- octanedioic acid
- octangular
- Octans
- octant
- octave
- Octavian
- octavo
- octet
- octette
- octillion
- October
- October 12
- October 24
- October Revolution
- Octoberfest
- octogenarian
- octonary
- octopod
- Octopoda
- Octopodidae
- octopus
- octoroon
- octosyllabic
- octosyllable
- octroi
- octuple
- ocular
- ocular muscle
- oculism
- oculist
- oculomotor
- oculomotor nerve
- oculopharyngeal muscular dystrophy
- oculus
- oculus dexter
- oculus sinister
- Ocyurus
- Ocyurus chrysurus