- old age
- old boy network
- olive branch
- olympiad
- olympian
- olympus mons
- omnivorousness
- on active service
- on and off
- on and on
- on behalf of
- on target
- on the market
- on the nose
- on the one hand
- on the part of done by
- 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
- onewho repeats what another says
- onion-skin
- onlooker
- only
- onomatoepic
- onomatopoeic
- onus
- onymous
- oogenesis
- oophorectomy
- openly
- operational
- ophthalmic
- ophthalmologist
- opine
- opinionative
- opium-den
- oracle
- orange tree
- orchestra man
- orchestrate
- ordeal
- orography
- orthodox
- orthopaedic
- oscillating
- osf
- out for
- out of bounds
- out of gear
- out of pocket expenses
- out of practice
- outing
- out of sorts
- out of spirits
- out of temper
- out of the straight
- outlay
- outpoint
- outrageous
- outside
- overbalance
- overcome
- overdue
- overturn
- overweight
- oxygen mask
- on balance
- oblivion
- on its last legs
- occupation
- off-key
- opposed
- order someone
- orientation
- over and above
- own up
- on the streets homeless
- overnight
- on the first instance
- outboard
- orientated
- open and above board
- occlussion
- onam
- obeisant
- oaten
- otherness
- on board
- ochery
- och
- off-wind
- oncology
- over toned
- of price
- optometry
- oblong pond
- old money