- on the safe side
- outstation
- oceanography
- olio
- old testament
- onanism
- on the first instance
- over hastiness
- one-twelfth
- old top
- offal pit
- on the table
- on the ropes
- on one mettle
- owe person a grudge
- on stilts
- olericulture
- on the record
- ohm
- old bachelor
- out wardens
- on its merits
- ooze from
- oversleep
- occlussion
- on parole
- one who has not performed the ceremony of tonsure
- optical scanner
- oast
- organic chemistry
- one who does the milching
- ordinary run of men
- overstaff
- obeisant
- ones right hand
- over head and ears
- outrank
- outbound
- old saying
- oaten
- one who feeds
- oviparous
- oedipus
- otherness
- oversimplify
- over the counter
- orgies
- outward things
- ontology
- oil squeezers
- one who dedicates
- onliness
- oozing latex
- onto
- opposite bank
- overexertion
- ominiscience
- owe to
- ostensibility
- one-way street
- octavo
- oncology
- on one's high horse
- orpiment
- odourless
- owner occupied
- of price
- on the stroke
- orbiculation
- octopod
- out in one's reckoning
- overgrazing
- outwear
- optometry
- oft-ignored
- oblong pond
- oersted
- ornithology
- one who toil ceaselessly to support his family
- old money
- objure
- on the stump
- out act
- oxygenation
- overexcite
- obscura
- of the first water
- oxide
- otto of roses
- odium
- oat
- oath
- obelisk
- observable
- obsession
- obstruction
- obviously
- occasionally
- odorous
- of no effect