- operating system
- operoseness
- opiate
- opiophagism
- opulence
- orange peel
- orangeade
- oratorical
- order
- option
- ordinary
- originated
- oriole
- ornament
- ornamental
- ornamentation
- overawed
- ornithic
- ortho
- orthograde
- orthopterology
- orthoptists
- oscillation
- ossicle
- out of date
- out of kilter
- on sight
- out of plumb
- outcast
- outdated
- outdo
- outer lid
- on patrol
- outfight
- outgoing
- outgoings
- outsmart
- ouzo
- oven
- over
- over against
- over bridge
- out of practice
- old age
- old boy network
- on credit
- on fall
- on occasion
- on one's hand
- on one's legs
- 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
- outing
- outcry
- outdoor
- outer
- outface
- outfitter
- outflow
- outgrowth
- outskirts
- outward