- on the files
- on the road
- on the scent
- on the spot
- on the strength of because of
- on the tip of one's tongue
- on view, on display
- once and for
- one another
- one place
- one roaming
- one which deserves to be imitated
- one who creates
- one who enjoys
- one who shaves
- one-act play
- one-man show
- ongoing
- onset
- oosperm
- opalesce
- opalescent
- open heart
- open letter
- open question
- open the floodgates
- open-handed
- opera-house
- operational guidelines
- operative
- operator
- 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
- outwardly
- outworn
- oval-shaped cassia
- ovation
- over fond
- over the hump
- overall
- overdone
- overeat
- overflowing the banks
- overhead
- override
- overseer
- oversexed
- overstatement
- overtake
- overworked
- on account of
- off duty
- out of order
- on one's guard
- on the hop
- on ice
- on the loose