- optimist
- optimistic
- ordinary fly
- ordinary man
- ordnance
- orectic
- oregano
- organiser
- oriental
- original
- ostentation
- osteomalacia
- ostracize
- ostrich policy
- otalgia
- otherwise
- out of sorts
- out of spirits
- out of temper
- out of the straight
- outlay
- outpoint
- outrageous
- outside
- overheads
- overindulge
- overpowering
- overreact
- oversea, overseas
- overturn
- overweight
- oxygen mask
- on account of
- off duty
- on one's guard
- on the hop
- once in a blue moon
- off one's nut
- oaf
- objection
- obstructive
- oddments
- offspring
- ogress
- old person
- omen
- omnipotence
- opprobrious
- organization
- originality
- outre
- over and over
- overdo
- overemphasize
- overt
- owing to
- own
- outine
- off the record
- on the spur of
- overhype
- oblative case
- oil-drum
- orthogonal
- on the way
- on the air
- opening sale
- ornithophily
- oscillated
- overdevelopment
- off line
- optical fiber
- oxylophyte
- only skin and bone
- one of the 18puranas
- organelle
- offset printing
- oar less
- oilery
- open the mouth
- ontogenesis
- over-age
- ones mouth
- open-hearted
- out door games
- ones own flesh and blood
- obstetrics
- opera
- order from the top
- out of heart
- oxidizable
- overlordship
- over plus
- orphanage
- outer rind
- ocean in
- office-seeker
- one who makes
- oleaginous ness
- of gestures