- obsessive-compulsive disorder
- obsessive-compulsive personality
- obsessively
- obsessiveness
- obsessivity
- obsidian
- obsolesce
- obsolescence
- obsolescent
- obsolete
- obsoleteness
- obstacle
- obstacle race
- obstetric
- obstetrical
- obstetrical delivery
- obstetrical toad
- obstetrician
- obstetrics
- obstinacy
- obstinance
- obstinate
- obstinately
- obstipate
- obstipation
- obstreperous
- obstreperously
- obstreperousness
- obstruct
- obstructed
- obstructer
- obstruction
- obstruction of justice
- obstructionism
- obstructionist
- obstructive
- obstructive shock
- obstructively
- obstructor
- obstruent
- obtain
- obtainable
- obtainment
- obtention
- obtrude
- obtrude upon
- obtrusive
- obtrusively
- obtrusiveness
- obtund
- obturate
- obturator
- obturator vein
- obtuse
- obtuse angle
- obtuse leaf
- obtuse triangle
- obtuse-angled triangle
- obtusely
- obtuseness
- obverse
- obviate
- obviating
- obviation
- obvious
- obviously
- obviousness
- oca
- ocarina
- o'casey
- occam
- occam's 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
- ockham's razor
- o'clock
- oconee bells
- o'connor
- 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