- oblivion
- old-time
- ossicle
- old testament
- omit
- outset
- omnifarous
- omnipresent
- onanism
- omnivorous
- opalize
- out of date
- own up
- on firm or solid ground
- on fire
- over bridge
- outgoings
- outsmart
- other-directed
- ogling
- on par with
- out of station
- on the streets homeless
- one shot
- on the anvil
- obelus
- open air
- on the ropes
- on the rack
- of course
- out of focus
- out of step
- on the turn
- on the understanding
- on the face of it
- on one mettle
- overriding
- oncoming
- onerous
- oriole
- one hundred thousand
- one man rule
- on cloud nine
- overcharge
- one who has taken an additional wife
- one who lives by his writing
- one yield
- ouzo
- one-handed
- oppugn
- outcast
- one-sided
- one-stop shop
- ones meat
- ones own
- only-begotten
- on average
- onus probandi
- open market
- ooze from
- open relationship
- on the wane
- on its merits
- oersted
- open and above board
- operate
- operatic
- operating system
- operoseness
- opiate
- out root
- opiophagism
- orange plant
- overindulgence
- opprobrium
- oppugnent
- opt out
- optant
- optic axis
- optic nerve
- overdress
- optimum
- opulence
- opposite view
- orange peel
- orangeade
- oratorical
- of one's own
- order
- oviferous
- out-take
- ordinary
- ordinary green colouring
- ordure
- organic analysis
- organic disease
- organza
- orient, orientate
- orientalist
- on active service