- Organ
- Organist
- Organizational
- Organogram
- Organza
- Orgy
- Orient
- Original
- Original sin
- Original tissue
- Originate
- Originates
- Originating
- Oriole
- Ornament
- Ornate
- Ornateness
- Ornery
- Ornis
- Ornithic
- Ornithologist
- Ornithology
- Orphanage
- Octal
- Orchestrics
- Orientate
- Osteoarthritis
- Osteopath
- Olio
- Orthodoxy
- Orthogonal
- Orthographic
- Orthopaedic
- Orthopedist
- Orthoptists
- Oryx
- Oscar
- Oscillating
- Osculate
- Ossification
- Ostenation
- Ostensible
- Ostensibly
- Osteotomy
- Ostracise
- Ostracism
- Other side
- Otiose
- Otto of roses
- Ought
- Ounce
- Our self
- Ourang-outang
- Ouster
- Out and away
- Out bargain
- Out measure
- Out of gear
- Out of hand
- Out of nowhere
- Out of ones own head
- Out of sight
- Out of the ordinary
- Out of the woods
- Out of thin air
- Out of touch
- Out root
- Out step
- Out wardens
- Out-herod
- Outback
- Outbid
- Outdistance
- Outdoor
- Outer door
- Outer skin
- Overdevelopment
- Outer wall
- Outermost
- Outfielder
- Outfitter
- Outgoing
- Outgrow
- Outine
- Outing
- Outlandish
- Outlast
- Outlaw
- Outlined
- Outnumber
- Outpatient
- Outpoint
- Outrank
- Outskirts
- Outsmart
- Outspokenness
- Outstay
- Outward show
- Ouzo
- Oval