- Opera-house
- Operational guidelines
- Operator
- Operculum
- Operetta
- Opex
- Opine
- Opium
- Opossum
- Opponent
- Opportuneness
- Opposite bank
- Opposite course
- Oppositely
- Oppositive
- Oppressiveness
- Oppressor
- Oppugnent
- Opt
- Opt out
- Optative
- Optic circle
- Optical
- Orange peel
- Optimistic
- Optimum
- Option
- Optionally
- Optometrist
- Optometry
- Opus
- Or what ever
- Or-else
- Oracle
- Oracular
- Orally
- Orange plant
- Oratorial powers
- Orbicular
- Orbit
- Order from the top
- Order of the day
- Ordinary fly
- Ordinary man
- Ordinary run of men
- Orexis
- Organelle
- Organic laws
- Organism
- Orientation
- Origami
- Ornamental
- Ornamental dot
- Ornamentation
- Ornamented
- Orotund
- Orphanize
- Orpiment
- Orris root
- Ort
- Orthograde
- Orthography
- Os
- Osmose
- Ostentation
- Osteoporosis
- Ostrich
- Ostrich policy
- Otalgia
- Otherness
- Otter
- Ourselves
- Out beat
- Out dare
- Out for
- Out of
- Out of danger
- Out of door
- Out of harm
- Out of order
- Out of pocket
- Out of the way
- Out put unit
- Out-of-date
- Out-of-tune sound
- Outbound
- Outbrave
- Outcaste
- Outcry
- Outdate
- Outdo
- Outdoor scenes
- Outdoors
- Outer cover
- Outer lid
- Outerly
- Outfight
- Outflow
- Outgo
- Outgrowth