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- O. J. Simpson
- Ö3 Austria Top 40
- Oahu
- Oak
- Oak Hill Cemetery (Lawrence, Kansas)
- Oak Ridge Associated Universities
- Oakland Alameda Coliseum
- Oakland Raiders
- Oakley, Kansas
- Oakwell
- Oarfish
- Oasis discography
- Oat
- Oath
- Oaxaca
- Oba Chandler
- Obadele Thompson
- Obazoa
- Oberallgäu
- Oberbergischer Kreis
- Obergefell v. Hodges
- Oberlin College
- Oberon (moon)
- Oberstdorf
- Obesity
- Obituary
- Object
- Object (grammar)
- Object-oriented programming
- Objectivism
- Objectivism (Ayn Rand)
- Objectivity
- Objectivity (journalism)
- Oblast
- Oblate spheroid
- Oblique order
- Oblique type
- Obock Region
- Oboe
- Observable universe
- Observation
- Obsessive-compulsive personality disorder
- Obsessive–compulsive disorder
- Obsidian
- Obstacle course
- Obstetric fistula
- Ocala, Florida
- Ocarina
- Očauš
- Occam's razor
- Occidental College
- Occitanie
- Occult
- Occupation
- Occupation of Czechoslovakia by Nazi Germany
- Occupational Safety and Health
- Occupational Safety and Health Administration
- Ocean
- Ocean acidification
- Ocean gyre
- Ocean surface wave
- Ocean thermal energy conversion
- Ocean's Eleven
- Ocean's Twelve
- Oceania Football Confederation
- Oceanic climate
- Oceanic dolphin
- Oceanic trench
- Oceanic whitetip shark
- Ocelot
- Ocicat
- Octal
- Octave
- October
- Octocorallia
- Octopoda
- Octopus
- Octopus's Garden
- Odawa
- Odenwald
- Oder River
- Odessa
- Odilon Redon
- Odin
- Odontoceti
- Odontochelys
- Odor
- Odyssey
- OECD
- Oestradiol
- Oestrous cycle
- Oettingen in Bayern
- Of Mice and Men
- Ofanto
- Off-road vehicle
- Off-site data protection
- Offa of Mercia
- Offa's Dyke
- Offensive guard
- Office
- Office suite
- Official residence
- Official U.S. PlayStation Magazine
- Offside rule
- Öfnerspitze
- Ofra Haza
- Ofterschwang
- Oganesson
- Ogawamachi Station (Tokyo)
- Ogdoad (Egyptian)
- Ögedei Khan
- Oggy and the Cockroaches
- OGPU
- Ogun River
- Ogyū Sorai
- Oh Se-hun
- Ohio County, West Virginia
- Ohio Senate
- Ohio State University
- Ohio Valley Conference
- Ohio Valley Wrestling
- Ohm's law
- Oil
- Oil paint
- Oil sands
- Ōita Prefecture
- Ojibwe
- OK
- Okara, Pakistan
- Okefenokee Swamp
- Okinawa
- Okinawa Prefecture
- Oklahoma
- Okroshka
- Oksana Baiul
- Oksana Grishuk
- Olaf I of Norway
- Ólafur Ragnar Grímsson
- Old Faithful Geyser
- Old Kingdom of Egypt
- Old Melbourne Gaol
- Old North Church
- Old Persian language
- Old Possum's Book of Practical Cats
- Old Prussian language
- Old Red Sandstone
- Old St Paul's Cathedral
- Old Style and New Style dates
- Old Synagogue (Erfurt)
- Old Testament
- Old Tom Morris
- Old World Underground, Where Are You Now?
- Old-growth forest
- Oldest dated rocks
- Oldham
- Oldham Athletic A.F.C.
- Olduvai Gorge
- Ole Kirk Christiansen
- Ole von Beust
- Olea capensis
- Oleander
- Oleg Tverdovsky
- Oleksiy Honcharuk
- Olfaction
- Olfactory bulb
- Olfactory system
- Olga Tañón
- Oligopoly
- Olinguito
- Olive (color)
- Olive tree
- Oliver & Company
- Oliver Cromwell
- Oliver Goldsmith
- Oliver Heaviside
- Oliver Postgate
- Oliver Smithies
- Oliver Wallace
- Oliver Wolcott
- Olivia Colman
- Olivia Newton-John
- Olly Murs
- Olmec
- Olof Palme
- Oloron-Sainte-Marie
- Olorotitan
- Olt River
- Olympia, Washington
- Olympic Games
- Om Puri
- Om Shanti Om
- Omagh
- Omar Sharif
- Omar Torrijos
- Omega Centauri
- Omega SA
- OMG (song)
- Omnivore
- On Human Nature
- On the Beach (novel)
- On the Floor
- On the Origin of Species
- On the Riviera
- On the Road
- Onam
- Once Sent from the Golden Hall
- Once Upon a Time in Mexico
- Once Were Warriors (movie)
- Oncilla
- Oncogene
- One Billion Rising
- One Direction
- One Hot Minute
- One in a Million (Aaliyah album)
- One Little Indian (movie)
- One Night Only
- One Piece
- One Unit Policy
- One-inch boy
- one-north MRT station
- One-party state
- One-Two-GO Airlines Flight 269
- Oneida County, Idaho
- OneRepublic
- Oneworld
- Oni
- Onion
- Online Computer Library Center
- Online Mendelian Inheritance in Man
- Online shopping
- Only Connect
- Ontario Hockey League
- Ontology
- Oobi
- Oodgeroo Noonuccal
- Oomycete
- Ooooooohhh... On the TLC Tip
- Oophorectomy
- Oort cloud
- Op art
- Opcode
- OPEC
- Opel
- Opel Combo
- Open All Hours
- Open bolt
- Open class
- Open content
- Open Handset Alliance
- Open Season (movie series)
- Open Season (movie)
- Open Season 2
- Open Season 3
- Open Shortest Path First
- Open star cluster
- Open University
- Open wheel car
- Openair Frauenfeld
- OpenBSD
- OpenOffice
- OpenOffice Base
- OpenOffice Calc
- OpenOffice Draw
- OpenOffice Impress
- OpenOffice Math
- OpenOffice Writer
- OpenVMS
- OpenVPN
- Opera
- Opera buffa
- Opera house
- Opera seria
- Opéra-Comique
- Operant conditioning
- Operating environment
- Operating system
- Operation Barbarossa
- Operation Entebbe
- Operation Mad Ball
- Operation Overlord
- Operation Restore Hope
- Operation Sea Lion
- Operation Uphold Democracy
- Operation Yewtree
- Operations and Checkout Building
- Operetta
- Opeth
- Ophelia (moon)
- Ophiolite
- Ophiuchus Supercluster explosion
- Ophthalmosauridae
- Ophthalmosaurus
- Opiliones
- Opioid
- Opium poppy
- Opium Wars
- Oppidum
- Oppositional defiant disorder
- Oppression
- Oprah Winfrey