- Omniscient
- Omnivorous
- On a full stomach
- On a level with
- On a platter
- On account of
- On approval
- On average
- On behalf of
- On firm or solid ground
- On parole
- On patrol
- On pins and needls
- On stilts
- On the carpet
- On the first instance
- On the go
- On the level
- On the right side of
- On the right side of forty
- On the round
- On the rove
- On the side of the grave
- On the streets
- On the tip of ones tongue
- On the understanding
- One who does the milching
- Onam
- Once in a while
- Oncologist
- One in a thousand
- One man rule
- One who earns
- One of the 18puranas
- One on one
- One shot
- One that is marching to destruction
- One track
- One way and another
- One who collects customs-tax
- One who gives joy
- One who has controlled passions
- One who indulges in insolence
- One who reads
- One who scratches
- Opposingly
- One with
- One's own flesh and blood
- One-fourth
- One-sided
- One-sidedly
- One-sidedness
- One-way
- Onerary
- Onerous
- Ones cup of tea
- Ones own flesh and blood
- Onewho repeats what another says
- Ongoing
- Onion-skin
- Onlooker
- Onlookers
- Only or on lie begetter
- Only skin and bone
- Onomatopoeia
- Ontology
- Onus probandi
- Onwards
- Oozing
- Opalesce
- Opaqueness
- Opeidoscope
- Open
- Open expanse of land
- Open handed
- Open market
- Open old wounds
- Open out
- Open persons eye
- Open source
- Open system
- Open verandha
- Open-hearted
- Open-mouthed
- Opened
- Openly
- Opera
- Operating expenditure
- Operating system
- Operatively
- Operator console
- Operose
- Ophthalmologist
- Opinion
- Opinionated
- Opponency
- Opportune
- Opportunism
- Opportunist
- Opposer