- ophthalmology
- opinionated
- opportune
- opportunism
- opposing
- oppressed
- optic circle
- optical instrument
- optimalise
- oral cavity
- orb
- organic farming
- organically
- organising
- organist
- orgy
- orifice
- original tissue
- originator
- ornis
- orotund
- ovary
- orthopedist
- oryza sativa
- ossification
- osteopath
- other-worldly
- ourang-outang
- out
- out blaze
- out of commission
- out of the picture
- out of this world
- outlandish
- outclass
- outdistance
- outlook
- outmaneuver
- outmoded
- outpouring
- outre'
- outright lie
- outsize
- outspread
- outstanding
- outworn
- oval-shaped cassia
- ovation
- over fond
- over the hump
- overall
- overdone
- overeat
- overflowing the banks
- overhead
- override
- overseer
- oversexed
- overstatement
- overtake
- overworked
- officinal
- officious
- oh save!
- oil nuts
- old man
- old-fashioned
- olent
- olfaction
- olibanum
- olivaceous
- ology
- ominous
- omnipresence
- on a par witih
- on cost
- on no account
- on someone's tail following
- on tap
- on the in crease
- on the nail
- on the rove
- on tow
- once more and again
- one in a thousand
- one who acts
- one who collects customs-tax
- one who erects
- one who gives joy
- one who ignores household duties as laid down by the scriptyres
- one who intimates
- onerousness
- one who reprimands
- onerary
- ones own affair
- one's own flesh and blood
- onslaught
- onyx
- oocyte
- oops-a-daisy