- Oft-used
- Oftentimes
- Ofttimes
- Oh for
- Oil
- Oil gas
- Oil of terminalia
- Oil pot
- Oil resource
- Oil sardine
- Oil spill
- Oil-colours
- Oil-crushers
- Oil-drum
- Oil-engine
- Oil-mill
- Oilman
- Old age home
- Old age pension
- Old as Adam
- Old clothes
- Old english
- Old saying
- Old sores
- Old testament
- Oldster
- Olent
- Olericulture
- Oligarch
- Oligarchy
- Oligopoly
- Olive
- Olympian
- Olympic
- Olympus mons
- Om
- Ombudsman
- Omelette
- Omen
- Omentums
- Ominiscience
- Ominous
- Omit from
- Omninescience
- On a shoe string
- On all fours
- On balance
- On cloud nine
- On edge
- On file
- On ones hand
- On record
- On sight
- On stand by
- On terms of
- On the road
- On the air
- On the edge
- On the ground
- On the gui vive
- On the in crease
- On the job training
- On the morrow of
- On the nail
- On the rack
- On the rocks
- On the skin
- On the spot
- On the straight
- On the stump
- On the verge of
- On the way out
- On the whole
- On the wrong track
- On thin ice
- Onanism
- Once
- Once bitten is twice shy
- Once in a life time
- Once more and again
- Oncology
- One after another
- One and the same
- One man
- One million
- One place
- One practised in adulation
- One roaming
- One short of
- One too many
- One which deserves to be imitated
- One who chants sama veda
- One who dedicates
- One who is inactive
- One who pities
- One who shaves
- One who submits
- One who throws
- One-man show
- One-mindedness