- of the first water
- oxide
- otto of roses
- odium
- object library
- obligate
- obliged
- obliquely
- obscene
- obscurantist
- obscuration
- occur
- occurrence
- ocra
- octagonal
- octahedron
- octolateral
- odeon
- of some sort
- of wide fame
- off
- off colour
- off kilter
- old stage
- old woman
- oleander
- 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
- on top of the world
- once and again
- once bitten, twice shy
- once in a life time
- once in a while
- one-size-fits-all
- one's pound of flesh
- open expanse of land
- open sesame
- open source
- opening
- oppositeness
- oppositive
- oppress
- oppressor
- optimist
- optimistic
- ordinary fly
- ordinary man
- ordnance
- orectic
- oregano
- organiser
- oriental
- original
- ostentation
- osteomalacia
- ostracize
- ostrich policy
- otalgia
- otherwise
- outcry
- outdoor
- outer
- outface
- outfitter
- outflow
- outgrowth
- outskirts
- outward
- outweigh
- ovarian
- ovaritis
- ovational
- over live
- overheads
- overindulge
- overpowering
- overreact
- oversea, overseas
- on average
- on balance
- on the face of it
- off the hook
- of the first magnitude
- of mark
- odds and ends
- one-time
- operation
- order someone
- orientation
- ostracism
- outlet
- overly
- out of service
- one shot