- oriental
- original
- out
- orography
- ourang-outang
- orthodox
- orthopaedic
- oscillating
- osf
- ostentation
- osteomalacia
- ostracize
- ostrich policy
- otalgia
- other-worldly
- overrun
- otherwise
- osteopath
- out for
- out of bounds
- ossification
- oryza sativa
- out of gear
- orthopedist
- ovary
- out of pocket expenses
- orthographic
- out of practice
- orotund
- outing
- ornis
- out of sorts
- out of spirits
- out of temper
- originator
- out of the straight
- original tissue
- outcry
- outdoor
- outer
- outface
- outfitter
- outflow
- outgrowth
- outlay
- ornate
- orifice
- orgy
- outpoint
- outrageous
- outside
- outskirts
- organist
- outward
- outweigh
- ovarian
- ovaritis
- ovational
- organising
- over live
- organically
- organic farming
- orb
- overbalance
- overcome
- overdue
- oral cavity
- overheads
- overindulge
- optimalise
- optical instrument
- optic circle
- oppressed
- overpowering
- opposing
- overreact
- oversea, overseas
- overturn
- opportunism
- overweight
- oxygen mask
- opportune
- opinionated
- ophthalmology
- originate
- operator
- operative
- operational guidelines
- opera-house
- open-handed
- open the floodgates
- open question
- open letter
- open heart
- opalescent
- opalesce
- oosperm
- onset
- ongoing
- originally