- old master
- old media
- old mine cut
- old money
- old moon
- old river
- old rose
- old saw
- old school
- old school tie
- old shoe
- old skool
- old sledge
- old sod
- old soldier
- old soul
- old squaw
- old stager
- old stamping ground
- old story, an
- old style
- old sweat
- old talk
- old wives' tale
- old woman
- old-boy network
- old-boyism
- old-established
- old-fashioned
- old-field colt
- old-girl network
- old-line
- old-line party
- old-maidish
- old-man cactus
- old-man's-beard
- old-man-and-woman
- old-time
- old-time dance
- old-timer
- old-timey
- old-womanish
- old-world
- olde-worlde
- olden
- older
- oldfangled
- oldie
- oldish
- oldspeak
- oldster
- oldwife
- oldy
- ole-
- olea
- oleaceous
- oleaginous
- oleander
- olearia
- oleaster
- oleate
- olecranon
- olefiant gas
- olefin
- olefin fiber
- olefin series
- olefine
- oleic
- oleic acid
- oleiferous
- olein
- oleo
- oleo oil
- oleo strut
- oleo-
- oleograph
- oleomargarine
- oleophilic
- oleoresin
- oleoresinous
- olericulture
- olestra
- olethreutid
- oleum
- oleyl alcohol
- olfaction
- olfactometer
- olfactometry
- olfactory
- olfactory bulb
- olfactory lobe
- olfactory nerve
- olfactory tract
- olibanum
- olicook
- olid
- oligaemia
- oligarch
- oligarchic
- oligarchy
- oligo-
- oligochaete
- oligoclase
- oligodendrocyte
- oligodontia
- oligogene
- oligomenorrhea
- oligomer
- oligomerous
- oligonucleotide
- oligopeptide
- oligophagous
- oligophrenia
- oligopoly
- oligopsony
- oligosaccharide
- oligospermia
- oligotrophic
- oligotrophy
- oliguria
- olingo
- olinguito
- olio
- olivaceous
- olivary
- olivary body
- olive
- olive branch
- olive brown
- olive crown
- olive drab
- olive family
- olive green
- olive oil
- olive shell
- olive-backed thrush
- olive-green
- olivenite
- olivette
- olivine
- olla
- olla podrida
- oller
- ollie
- olm
- ologoan
- ology
- ololiuqui
- oloroso
- olpe
- olykoek
- olé
- om
- omadhaun
- omakase
- omasum
- omber
- ombre
- ombrellino
- ombro-
- ombrogenous
- ombrophilous
- ombrophobous
- ombu
- ombudsman
- ombudsperson
- ombudswoman
- omega
- omega baryon
- omega meson
- omega minus
- omega-3 fatty acid
- omega-c baryon
- omega-minus particle
- omelet
- omelette
- omen
- omentum
- omeprazole
- omer
- omertà
- omicron
- omics
- omigod
- ominous
- omissible
- omission
- omissive
- omit
- omitted
- ommateum
- ommatidium
- ommatophore
- omn. bih.
- omn. hor.
- omn. man.
- omn. noct.
- omn. quadr. hor.
- omni-
- omnia mutantur, nos et mutamur in illis
- omnia vincit amor
- omnibearing
- omnibus
- omnibus clause
- omnicide
- omnicompetent
- omnidirectional
- omnidistance
- omnifarious
- omnific
- omnificent
- omnigraph
- omnipotence
- omnipotent
- omnipresent
- omnirange
- omniscience
- omniscient
- omnisexual
- omnishambles
- omnium-gatherum
- omnivore
- omnivorous
- omophagia
- omophorion
- omphacite
- omphalos
- omphaloskepsis
- on
- on a dime
- on a first-name basis
- on a limb
- on a par with
- on a pedestal, put
- on a rampage
- on a roll
- on a shoestring
- on a string
- on a tangent
- on account
- on account of
- on again
- on all fours
- on an even keel
- on and off
- on and on
- on approval
- on balance
- on behalf of
- on bended knee
- on board
- on borrowed time, live
- on call
- on camera
- on cloud nine
- on commission
- on condition that
- on consignment
- on deck
- on demand
- on dit
- on draft
- on duty
- on earth
- on easy street
- on edge
- on end
- on faith, take it
- on file
- on fire
- on foot
- on good terms
- on hand
- on high
- on hold
- on ice
- on in years
- on its merits
- on key
- on line
- on message
- on no account
- on occasion
- on one hand
- on one's
- on one's account
- on one's behalf
- on one's best behavior
- on one's doorstep
- on one's feet
- on one's good behavior
- on one's guard
- on one's head
- on one's heels
- on one's high horse
- on one's home ground
- on one's honor
- on one's last legs
- on one's mind
- on one's own