- ochre
- octa-
- octachord
- octad
- octagon
- octagonal
- octahedral
- octahedron
- octameter
- octane
- octangular
- octant
- octaroon
- octastyle
- octateuch
- octavalent
- octave
- octavo
- octennial
- octet
- october
- octogenarian
- octonary
- octopod
- octopus
- octoroon
- octosyllabic
- octosyllable
- octroi
- octuple
- ocular
- oculist
- oculo-
- oculonasal
- od
- odalisque
- odd
- oddities
- oddity
- oddly
- odds
- ode
- odious
- odium
- odometer
- odontalgia
- odontoid
- odontology
- odor
- odoriferous
- odorless
- odorous
- odyssey
- oecumenical
- oedema
- oesophagus
- of
- off
- offal
- offcut
- offence
- offend
- offended
- offender
- offending
- offense
- offensive
- offer
- offered
- offering
- offertories
- offertory
- offhand
- office
- officeholder
- officer
- officered
- officering
- official
- officialism
- officially
- officiary
- officiate
- officiated
- officiating
- officinal
- officious
- offing
- offish
- offset
- offsetting
- offshoot
- offshore
- offspring
- oft
- often
- oftentimes
- ofttimes
- ogle
- ogled