- occasionally
- occur
- oldish
- occurrence
- ocra
- octagonal
- octahedron
- octolateral
- ones meat
- odeon
- odorous
- ones own
- only-begotten
- oak
- oxygenize
- of no effect
- onus probandi
- onus
- of some sort
- open market
- ophite
- on one's own solitary
- of wide fame
- off
- off colour
- off kilter
- ovicular
- overindulge
- off-road
- offbeat
- overheads
- offensive
- operate
- oppositeness
- oil bottle
- oil of terminalia
- old age
- old boy network
- operoseness
- old stage
- old woman
- opiate
- out root
- operational
- oleander
- olive branch
- olympiad
- olympian
- olympus mons
- omnivorousness
- on active service
- on and off
- on and on
- on behalf of
- on credit
- opt out
- on fall
- optant
- on occasion
- on one's hand
- on one's legs
- optic nerve
- on target
- overdress
- optimum
- obstacle race
- opulence
- on the market
- on the nose
- on the one hand
- orange peel
- oversimplification
- on the part of done by
- orangeade
- oratorical
- overpayment
- of one's own
- on purpose
- order
- on top of the world
- oviferous
- once and again
- once bitten, twice shy
- once in a life time
- once in a while
- ordinary
- ordinary green colouring
- one who eats
- one who is inactive
- one who is not done his duties
- one who reads
- one who sleeps at sunrise
- one who throws
- one-size-fits-all
- organza
- off-time
- orient, orientate
- one's pound of flesh
- orientalist
- one rate