- out dare
- oath taking
- oyster-catcher
- obscene language
- ontological
- oxygenize
- old wine in a new bottle
- oculus
- owlet
- on the skin
- old before one time
- operant
- oppressing others
- oviducal
- old wares
- owl
- one who earns
- oneirology
- ootheca
- opposite side
- oscitant
- olympus
- on request
- omni volume
- optative
- original sin
- octopede
- off one's hand
- override, overrule
- of old standing
- on the round
- overgrowth
- osteology
- octagon
- osteitis
- over haste
- outstay
- oligarch
- optional
- obituary
- oblige
- oblivious
- obnoxious
- occult
- occupancy
- occupant
- octahedral
- october
- odd work
- odic
- of sorts
- of the first rank
- of your please
- off one's dot
- off-day
- off-putting
- offby heart
- offerings of flowers
- office hunter
- official
- old english
- old rags
- olfactory
- on duty
- on offer up
- on the files
- on view, on display
- once and for
- one another
- one place
- one roaming
- one which deserves to be imitated
- one who creates
- one who enjoys
- one who shaves
- ongoing
- onset
- oosperm
- opalesce
- opalescent
- opera-house
- operational guidelines
- operative
- operator
- ophthalmology
- opinionated
- opportune
- opportunism
- oral cavity
- orb
- organic farming
- organically
- organising
- organist
- orgy
- orifice
- original tissue
- ossification
- osteopath
- other-worldly