- Obscuration
- Obscurity
- Observance
- Observantly
- Observation
- Observe
- Observing silence
- Obsessive
- Obsidian
- Obsolescence
- Obstetrics
- Obstinacy
- Obstinately
- Obstructing
- Obstruction
- Obtrusion
- Obturate
- Obtuse
- Obtuse angle
- Occasional
- Occlussion
- Occult
- Occultism
- Occupational disease
- Occurs
- Ocherous
- Ocimum tenuiflorum
- Octagonal
- Octahedral
- Octahedron
- Octennial
- Octet rule
- Octo-
- Octopus
- Od
- Odd man
- Odd work
- Oddity
- Odiousness
- Odium
- Odontology
- Odoriferous
- Odorous
- Oeuvre
- Of a low standard
- Of an even temper
- Of gestures
- Of god's
- Of no effect
- Of note
- Of old standing
- Of one and one only
- Of ones own volition
- Of so little
- Of the left
- Of the moon's
- Of wide fame
- Of words
- Off duty
- Off line
- Off ones hand
- Off ones onion
- Off ramp
- Off scoring
- Off the point
- Off the track
- Off with his head
- Off-day
- Off-street
- Offensive
- Offerings
- Official dom
- Officially
- Officials
- Ogling
- Oh save!
- Ohm
- Oil colour
- Oil nuts
- Oil-skin
- Oiling
- Oily grain
- Olam
- Old age
- Old hand
- Old man
- Old rags
- Old stage
- Old story
- Old wares
- Old wine in a new bottle
- Old wisdom
- Old woman
- Older
- Oleander
- Oleograph
- Olfaction
- Ology
- Olympiad
- Olympus