- oceanography
- ocelot
- observance
- obtainable
- obtrude
- occupational therapy
- octangular
- octave
- octavo
- octette
- october
- octonary
- octopod
- octroi
- obverse
- obvious
- occasion
- occidental
- occupancy
- occupational hazard
- octad
- octagonal
- octal
- octane number
- omnipresence
- overturn
- oodles
- omega
- ox
- out of sight
- optic
- ophthalmic
- open
- oxidize
- of old
- orientalist
- on the wane
- overlay
- outfit
- outflank
- outdo
- outstrip
- old
- ovoid
- old woman
- osmium
- outcry
- origin
- out
- openness