- onewho repeats what another says
- onion-skin
- oriole
- onlooker
- only
- onomatoepic
- onomatopoeic
- onus
- onymous
- oogenesis
- oophorectomy
- old buffer
- open expanse of land
- ornithic
- ortho
- official despatches
- open sesame
- open source
- orthopterology
- osteoporosis
- orthoptists
- opening
- oscillation
- openly
- operational
- ophthalmic
- ophthalmologist
- opine
- opinionative
- outside work
- opium-den
- on the right side of forty
- oppositeness
- oppositive
- oppress
- oppressor
- oust
- optimist
- optimistic
- out and away
- oracle
- orange tree
- orchestra man
- orchestrate
- ordeal
- out of kilter
- ordinary fly
- ordinary man
- ordnance
- orectic
- oregano
- organiser
- out of station
- oriental
- original
- out of step
- orography
- oncologist
- orthodox
- orthopaedic
- oscillating
- osf
- ostentation
- osteomalacia
- ostracize
- ostrich policy
- otalgia
- outcast
- outdated
- otherwise
- outdo
- orang-outang, orangutan
- out for
- out of bounds
- osteoarthritis
- overprint
- out of gear
- on all fours
- octane
- out of pocket expenses
- outfight
- out of practice
- outgoing
- outing
- outgoings
- out of sorts
- out of spirits
- out of temper
- outpour
- out of the straight
- overwhelmingly strong
- octangular
- outcry
- outdoor
- outer
- outface
- outfitter
- outflow
- outgrowth
- old man