- okta
- Oktoberfest
- Okuninushi
- OL
- Ol.
- Olaf I
- Olaf II
- Olaf V
- olallieberry
- Öland
- Olathe
- Olav V
- Olbers
- Olbers' paradox
- Olcott
- old
- old Adam
- old age
- old age pension
- old as Adam
- Old Bailey
- Old Believer
- Old Bill
- old bird
- old boy
- old-boyism
- old boy network
- old-boy network
- Old British
- Old Bulgarian
- Old Castile
- Oldcastle
- Old Catholic
- old chap
- old chestnut
- old chum
- Old Church Slavonic
- old clothes man
- old college try, the
- Old Comedy
- Old Contemptibles
- old country
- old covenant
- Old Curiosity Shop, The
- Old Danish
- Old Dart
- Old Delhi
- Old Dominion
- Old Dutch
- olden
- Oldenbarneveldt
- Oldenburg
- Old English
- Old English pattern
- Old English sheepdog
- older
- old-established
- olde-worlde
- old face
- Old Faithful
- oldfangled
- old fart
- old fashioned
- old-fashioned
- old fellow
- Oldfield
- old field
- old-field colt
- Old Flemish
- old fogy
- Old Franconian
- Old French
- Old Frisian
- old fustic
- old girl
- old-girl network
- Old Glory
- old goat
- old gold
- old growth
- old guard
- Old Guard
- Old Guardism
- old habits die hard
- Oldham
- old hand
- Old Harry
- old hat
- Old High German
- Old Icelandic
- old identity
- oldie
- Old Ionic
- Old Iranian
- Old Irish
- Old Ironsides
- oldish
- Old Italian
- Old Kingdom
- old lady
- Old Lady of Threadneedle Street
- Old Latin
- old-line
- old-line party
- Old Line State
- Old Low Franconian
- Old Low German
- old maid
- old-maidish
- old man
- old man's beard
- old-man's-beard
- Old Man and the Sea, The
- old-man-and-woman
- old-man cactus
- Old Man of the Sea
- old man of the woods
- old master
- old media
- old mine cut
- old money
- old moon
- Old Nick
- Old Norse
- Old North French
- Old Northwest
- Old Norwegian
- Old Orchard Beach
- Oldowan
- Old Persian
- Old Portuguese
- Old Pretender
- Old Provençal
- Old Prussian
- Old Red Sandstone
- Old Ritualist
- old river
- old rose
- Old Russian
- Olds
- old saw
- Old Saxon
- old school
- old school tie
- Old Scratch
- old shoe
- old skool
- Old Slavic
- Old Slavonic
- old sledge
- old sod
- old soldier
- Old soldiers never die; they only fade away
- old soul
- Old South
- Old South Arabian
- Old Spanish
- Old Spanish Trail
- oldspeak
- old squaw
- old stager
- old stamping ground
- oldster
- Old Stoa
- Old Stone Age
- old story, an
- old style
- Old Style
- old sweat
- Old Swedish
- old talk
- Old Test.
- Old Testament
- old-time
- old-time dance
- old-timer
- old-timey
- Old Turkic
- Olduvai Gorge
- Old Welsh
- Old West
- oldwife
- Old Wives' summer
- old wives' tale
- old woman
- old-womanish
- Old World
- old-world
- Old World monkey
- oldy
- OLE
- olé
- ole-
- olea
- oleaceous
- oleaginous
- Olean
- oleander
- olearia
- oleaster
- oleate
- olecranon
- OLED
- olefiant gas
- olefin
- olefine
- olefin fiber
- olefin series
- oleic
- oleic acid
- oleiferous
- olein
- Olekma
- Olenek
- oleo
- oleo-
- oleograph
- oleomargarine
- oleo oil
- oleophilic
- oleoresin
- oleoresinous
- oleo strut
- olericulture
- olestra
- olethreutid
- oleum
- O level
- oleyl alcohol
- olfaction
- olfactometer
- olfactometry
- olfactory
- olfactory bulb
- olfactory lobe
- olfactory nerve
- olfactory tract
- OLG
- Olga
- olibanum
- olicook
- olid
- oligaemia
- oligarch
- oligarchic
- oligarchy
- oligo-
- Oligocene
- oligochaete
- oligoclase
- oligodendrocyte
- oligodontia
- oligogene
- oligomenorrhea
- oligomer
- oligomerous
- oligonucleotide
- oligopeptide
- oligophagous
- oligophrenia
- oligopoly
- oligopsony
- oligosaccharide
- oligospermia
- oligotrophic
- oligotrophy
- oliguria
- Ólimbos
- Olimpico
- Olin
- Olinda
- olingo
- olinguito
- olio
- Oliphant
- olivaceous
- Olivares
- olivary
- olivary body
- olive
- Olive
- olive-backed thrush
- olive branch
- olive brown
- olive crown
- olive drab
- olive family
- olive green
- olive-green
- olivenite
- olive oil
- Oliver
- Oliver Twist
- Olives
- olive shell
- olivette
- Olivia
- Olivier
- Olivier, Laurence
- olivine
