- OK
- Oklahoma
- okra
- old
- old age
- old age pension
- old age pensioner
- Old Bailey
- old flame
- old hat
- old maid
- old master
- old people's home
- Old Testament
- old wives' tale
- Old World
- olden
- old-fashioned
- old-time
- old-timer
- oligarchy
- olive
- olive green
- olive oil
- Olympic
- Olympic Games
- OM
- Oman
- OMB
- ombudsman
- omelette
- omen
- ominous
- ominously
- omission
- omit
- omnibus
- omnipotence
- omnipotent
- omnipresent
- omniscient
- omnivorous
- on
- ON
- ONC
- once
- once-over
- oncoming
- OND
- one
- one-armed bandit
- one-liner
- one-man
- one-man band
- one-night stand
- one-off
- one-on-one
- one-parent family
- one-piece
- onerous
- one's
- oneself
- one-sided
- onetime
- one-to-one
- one-touch dialling
- one-upmanship
- one-way
- ongoing
- onion
- online
- online banking
- online shopping
- onlooker
- only
- onrush
- on-screen
- onset
- onshore
- onside
- onslaught
- Ont.
- Ontario
- on-the-job
- on-the-spot
- onto
- onus
- onward
- onwards
- onyx
- oodles
- ooh
- oops
- ooze
- opacity
- opal
- opaque
- OPEC
- open
- open day
- open letter
- open market
- open marriage
- open prison
- open sandwich
- open season
- open shop
- Open University
- open verdict
- open-air
- open-and-shut
- opencast
- open-ended
- opener
- open-handed
- open-heart surgery
- openhearted
- opening
- opening hours
- opening night
- opening time
- openly
- open-minded
- open-mouthed
- open-necked
- openness
- open-plan
- opera
- opera glasses
- opera house
- opera singer
- operate
- operatic
- operating room
- operating system
- operating theatre
- operation
- operational
- operative
- operator
- operetta
- ophthalmic optician
- ophthalmologist
- opinion
- opinion poll
- opinionated
- opium
- opponent
- opportune
- opportunism
- opportunist
- opportunity
- oppose
- opposed
- opposing
- opposite
- opposite number
- opposite sex
- opposition
- oppress
- oppressed
- oppression
- oppressive
- oppressor
- opprobrium
- opt
- optic
- optical
- optical character reader
- optical character recognition
- optical fibre
- optical illusion
- optician
- optimism
- optimist
- optimistic
- optimize
- optimum
- option
- optional
- opulence
- opulent
- opus
- or
- OR
- oracle
- oral
- orally
- orange
- orange blossom
- orange juice
- orange tree
- orangeade
- Orangeman
- orangutang
- oration
- orator
- oratorio
- oratory
- orb