- on cost
- on no account
- on someone's tail following
- on tap
- on the in crease
- on tow
- once more and again
- one in a thousand
- ossify
- ostenation
- ostentatious
- ostrich
- otorhinolaryngology
- ours
- out of the ordinary
- out of the question
- out roar
- out-of-tune sound
- outbid
- outbreak
- outburst
- outlier
- outlined
- outlying
- outmanoeuvre
- outnumbered
- output
- outshine
- oval
- oval-shaped
- overjoyed
- overlay
- overlook
- overproduction
- overrate
- overrun
- overshadow
- overstate
- on average
- on balance
- on the beam
- on behalf of someone, on someone's behalf
- on cloud nine
- of course
- out of doors
- overindulgence
- on the face of it
- off the hook
- on the horizon
- of the first magnitude
- on its last legs
- of mark
- oblation
- odd
- obsess
- occupation
- odds
- offender
- offering
- off-key
- on
- open air
- operation
- opinion
- opposed
- ordain
- order someone
- ordinance
- organism
- orientation
- ostensibly
- ostracism
- outlet
- overact
- outright
- over and above
- obligatory
- ornamental
- ornamentation
- ornithic
- ortho
- orthograde
- orthopterology
- orthoptists
- oscillation
- ossicle
- out of date
- out of kilter
- out of plumb
- outcast
- outdated
- outdo
- outer lid
- outfight
- outgoing
- outgoings
- over zeal
- overawe
- overcast
- overcharge