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Articles beginning with O
- Oni
- Oni (folklore)
- Onifai
- Oniferi
- Ōnin
- Ōnin War
- Onion
- Onionhead
- Onley, Virginia
- Online and offline
- Online game
- Online Mendelian Inheritance in Man
- Online pharmacy
- Online shopping
- Only Connect
- Only Fools and Horses
- Only Girl (In the World)
- Only Human
- Onnens
- Ōnojō
- Onomastics
- Onomatopoeia
- Onon (river)
- Onondaga County, New York
- Onryō
- ONS coding system
- Onsernone
- Onslow County, North Carolina
- Onslow, Iowa
- Onslow, Nova Scotia
- Ontario
- Ontario County, New York
- Ontario Highway 10
- Ontario Highway 101
- Ontario Highway 11
- Ontario Highway 118
- Ontario Highway 12
- Ontario Highway 144
- Ontario Highway 17
- Ontario Highway 21
- Ontario Highway 23
- Ontario Highway 24
- Ontario Highway 26
- Ontario Highway 28
- Ontario Highway 33
- Ontario Highway 34
- Ontario Highway 4
- Ontario Highway 400
- Ontario Highway 401
- Ontario Highway 402
- Ontario Highway 407
- Ontario Highway 409
- Ontario Highway 410
- Ontario Highway 416
- Ontario Highway 417
- Ontario Highway 5
- Ontario Highway 6
- Ontario Highway 62
- Ontario Highway 7
- Ontario Highway 8
- Ontario Highway 89
- Ontario Highway 9
- Ontario Hockey League
- Ontario Lacus
- Ontario Liberal Party
- Ontario New Democratic Party
- Ontario, California
- Ontario, Ohio
- Ontario, Oregon
- Ontology
- Ontonagon County, Michigan
- Onward
- Onward (movie)
- Onyx
- Onyx River
- Oobi
- Oodgeroo Noonuccal
- Oodnadatta, South Australia
- Ooid
- Oologah, Oklahoma
- Oomycete
- Oona Chaplin
- Oona O'Neill
- Ooooooohhh... On the TLC Tip
- Oophorectomy
- Oops!... I Did It Again
- Oops!... I Did It Again (song)
- Oort cloud
- Oost Gelre
- Oostende
- Oosterscheldekering
- Oosterzele
- Oostkamp
- Oostrozebeke
- Ooststellingwerf
- Oostzaan
- Ooty
- Op art
- Opa-locka, Florida
- Opabinia
- Opacity
- Opal
- Opcode
- OPEC
- Opel
- Opel Adam
- Opel Agila
- Opel Antara
- Opel Astra
- Opel Cascada
- Opel Combo
- Opel Corsa
- Opel Insignia
- Opel Karl
- Opel Meriva
- Opel Mokka
- Opel Omega
- Opel Signum
- Opel Sintra
- Opel Vectra
- Opel Vivaro
- Opel Zafira
- Opelika, Alabama
- Opelousas, Louisiana
- Open All Hours
- Open bolt
- Open class
- Open content
- Open Database License
- Open Door Policy
- Open Handset Alliance
- Open license
- Open Road
- Open Season
- Open Season (1974 movie)
- Open Season (movie series)
- Open Season (movie) 2006 animated-comedy movie
- Open Season 2
- Open Season 3
- Open Shortest Path First
- Open Site
- Open source
- Open star cluster
- Open University
- Open University of Israel
- Open wheel car
- Open world
- Openair Frauenfeld
- OpenBSD
- OpenDocument
- OpenGL
- Opening act
- OpenOffice
- OpenOffice Base
- OpenOffice Calc
- OpenOffice Draw
- OpenOffice Impress
- OpenOffice Math
- OpenOffice Writer
- OpenSolaris
- OpenStep
- OpenStreetMap
- openSUSE
- OpenVMS
- OpenVPN
- OpenWrt
- OpenZaurus
- Opera
- Opera (web browser)
- Opera buffa
- Opéra de Monte-Carlo
- Opera house
- Opera seria
- Opera Software
- Opéra-Comique
- Operación Puerto doping case
- Operand
- Operant conditioning
- Operating environment
- Operating system
- Operation
- Operation (mathematics)
- Operation Barbarossa
- Operation Boatswain Memorial
- Operation Car Wash
- Operation Cornflakes
- Operation Crossbow
- Operation Cyclone
- Operation Downfall
- Operation Eagle Claw
- Operation Entebbe
- Operation Ground and Pound
- Operation Mad Ball
- Operation Meetinghouse
- Operation Mincemeat
- Operation Nickel Grass
- Operation Overlord the 1944 campaign for the invasion of continental Europe in World War II
- Operation Restore Hope
- Operation Sea Lion
- Operation Searchlight
- Operation Uphold Democracy
- Operation Yewtree
- Operations and Checkout Building
- Operetta
- Opeth
- Opfenbach
- Opfertshofen
- Opfikon
- Opglabbeek
- Ophelia (moon)
- Ophidiophobia
- Ophiolite
- Ophiotaurus
- Ophiuchus
- Ophiuchus Supercluster explosion
- Ophthalmology
- Ophthalmosauridae
- Ophthalmosaurus
- Opiliones
- Opinion
- Opinion poll
- Opio, Alpes-Maritimes
- Opioid substances that act on opioid receptors to produce morphine-like effects
- Opisthocoelicaudia
- Opisthokont
- Opium
- Opium poppy
- Opium Wars
- Opmeer
- Opole
- Opole Voivodeship
- Opossum
- Opp, Alabama
- Oppède
- Oppens
- Oppenweiler
- Oppidum
- Oppligen
- Opportunity (rover)
- Opportunity cost
- Opposite
- Opposite number
- Opposition (astronomy and astrology)
- Oppositional defiant disorder
- Oppression
- Oppuurs
- Oprah Winfrey
- Oprichnina
- Opsin
- Opsterland
- Opt-out
- Optare Alero
- Optare CityPacer
- Optare MetroRider
- Optare Solo
- Optare Soroco
- Optic chiasma
- Optic nerve
- Optic tectum
- Optic tract
- Optical character recognition
- Optical depth
- Optical disc drive
- Optical fiber thin fiber of glass or plastic that can carry light
- Optical filter
- Optical gain medium
- Optical illusion
- Optical printer
- Optics
- Optima
- Optimism
- Optimization (computer science)
- Optimus Prime
- Opus Dei
- Opus number
- Opwijk
- Oquawka, Illinois
- OR gate
- Ora (Rita Ora album)
- Ora Namir
- Oraàs
- Oracle
- Oracle Corporation
- Oracle Database
- Oracle of Delphi
- Oracle Park
- Oradea
- Orai
- Orain
- Orainville
- Orakzai District
- Oral cancer
- Oral history
- Oral law
- Oral rehydration therapy
- Oral sex
- Oral tradition
- Oran
- Oran Province
- Orange