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- O'Brian, Patrick
- O'Hara, Frank
- O(renthal) J(ames) Simpson
- O-Kagami
- OK MR (film)
- OK Mister (film)
- OS/2
- OSF
- OSI model
- O\'Brien, Dan Dio
- O\'Brien, Flann
- O\'Brien, Kate
- O\'Brien, Margaret
- O\'Brien, William
- O\'Brien, Willis
- O\'Casey, Sean
- O\'Connell, Daniel
- O\'Conner, Rory
- O\'Connor, Flannery
- O\'Connor, Frank
- O\'Connor, Thomas
- O\'Farrell, Frank
- O\'Flaherty, Liam
- O\'Hara, John
- O\'Higgins, Bernardo
- O\'Keeffe, Georgia
- O\'Keeffe, John
- O\'Neal, Shaquille
- O\'Neill, Eugene
- O\'Neill, Hugh
- O\'Reilly, Tony
- O\'Sullivan, Sonia
- O\'Toole, Peter
- O\'brien, William Smith
- O Dalaigh, Cearbhall
- O Henry
- O Iran (movie)
- O ano da morte de Ricardo Reis
- O kapetan mikhalis
- O sia La scuola degli amanti Cosi fan tutte
- Oahu
- Oak
- Oak Ridge
- Oakland
- Oamaru
- Oarfish
- Oarweed
- Oasis
- Oast house
- Oat
- Oates, Joyce Carol
- Oates, Titus
- Oaxaca (city)
- Oaxaca (state)
- Ob
- Obafemi Awolowo
- Obama, Barack Hussein
- Obasanjo, Olusegun
- Obbligato
- Obed Hussey
- Obedience
- Obelisk
- Oberhausen
- Obermaier, Hugo
- Oberon, Merle
- Oberon (astrology)
- Oberon (literature)
- Obesity
- Object-oriented design
- Object-oriented programming
- Object Code
- Object relations
- Objective
- Oblako v Stanah
- Obligation
- Oblomov
- Oboe (army)
- Oboe (music)
- Obote, Milton
- Obraztsov, Sergei
- Obregon, Alvaro
- Obrenovich
- Obscenity law
- Observation
- Observatory
- Observer
- Obsession
- Obsidian
- Obsidian hydration-rim dating
- Obuchi, Keizo
- Occasionalism
- Occluded front
- Occult sciences
- Occultation
- Occupational disease
- Occupational psychology
- Occupational therapy
- Ocean
- Ocean Drilling Program
- Ocean current
- Ocean trench
- Oceanarium
- Oceanids
- Oceanography
- Oceanus
- Ochoa, Severo
- Ockham, William of
- Ockley, Simon
- Octahedron, regular
- Octane rating
- Octans
- Octatonic scale
- Octave
- Octave Cremazie
- Octave Feuillet
- Octave Henri Marie Mirbeau
- Octave Victor Houdas
- Octavia (1th century BC)
- Octavia (40-62 AD)
- Octavian Goga
- Octavio Paz
- Octet
- Octet rule
- Octo basse
- October
- Octobrists
- Octopus
- Ocussi Ambeno
- Oda, Nobunaga
- Odaenathus, Septimius
- Odd Hassel
- Odd number
- Oddsson, David
- Ode
- Odense
- Odenwald
- Oder
- Oder-Neisse Line
- Odessa
- Odets, Clifford
- Odette Marie Celine Hallowes
- Odilon Redon
- Odin
- Odin den\' Ivana Denisovica
- Odinga, Oginga
- Odoacer
- Odon Horvath
- Odonata
- Odrerir
- Odysseus (book)
- Odysseus (mythology)
- Odysseus Elytis
- Oe, Kenzaburo
- Oedema
- Oedipus
- Oedipus at Colonus
- Oedipus complex
- Oedipus the King
- Oehlenschlager, Adam Gottlob
- Oenone
- Oersted
- Oersted, Hans Christian
- Oerter,Al(fred) Adolf
- Oesophagus
- Oestrogen
- Oestrus
- Of Human Bondage
- Of Mice and Men
- Offa\'s Dyke
- Offaly
- Offenbach
- Offenbach, Jacques
- Offenbach am Main
- Offenburg
- Offer
- Office automation
- Office of Strategic Services
- Offline
- Offline editing
- Offset printing
- Offside (Iranian film)
- Ofonius Tigellinus
- Ogaden
- Ogata Kenzan
- Ogata Korin
- Ogbomosho
- Ogilby, John
- Oginga Odinga
- Ogino, Ginko
- Oglethorpe, James
- Ogoni
- Ogooué
- Ogun
- Ohio (river)
- Ohio (state)
- Ohlin, Bertil
- Ohm
- Ohm, Georg Simon
- Ohm\'s law
- Ohmic heating
- Ohonamochi
- Ohrid
- Ohrid (lake)
- Oil
- Oil crop
- Oil of almond
- Oil paint
- Oil palm
- Oil refinery
- Oil spill
- Oilbird
- Oils
- Oimyakon
- Oise
- Oise (department)
- Oistrakh, David
- Oita
- Oka
- Okapi
- Okatakyie Afrifa
- Okavango
- Okavango Delta
- Okayama
- Okhotsk
- Oki
- Okinawa
- Oklahoma
- Oklahoma City
- Okra
- Okri, Ben
- Oktyabrski
- Okubo, Toshimichi
- Okuma, Shigenobu
- Ola Hansson
- Olaf Bull
- Olaf I
- Olaf II
- Olaf Stapledon
- Olaf V
- Olafchristian Birkeland
- Olah, George
- Oland
- Olaudah Equiano
- Olaus Celsius
- Olaus Roemer
- Olbers, Heinrich
- Olbers\'s paradox
- Olbia
- Olbrich, Joseph
- Old Believers
- Old Catholic
- Old English sheepdog
- Old King Cole
- Old Man of the Sea
- Old Trafford
- Old World
- Oldenbarneveldt, Jahn van
- Oldenburg
- Oldenburg, Claes Thure
- Oldfield, Anne
- Oldfield, Barney
- Oldham
- Oldman, Garry
- Olds, Ransom Eli
- Olduvai Gorge
- Oleander
- Olearius, Adam
- Oleg
- Oleg Akimoshkin
- Olegario Victor Andrade
- Oleron
- Olga, St
- Olga Ivanovna Smirnova
- Olga Korbut
- Oligarchy
- Oligocene epoch
- Oligochaeta
- Oligodendrocyte
- Oligopoly
- Oligosaccharide
- Olin Jenck Eggen
- Oliphant, Mark
- Olivares, Count-Duke of
- Olive
- Olive Emilie Schreiner
- Olive branch
- Oliveira, Manoel de
- Olivenite
- Oliver, Isaac
- Oliver Cromwell
- Oliver Evans
- Oliver Goldsmith
- Oliver Heaviside
- Oliver La Farge
- Oliver Lodge
- Oliver St John Gogarty
- Oliver Stone
- Oliver Tambo
- Oliver Twist
- Oliver Wendell Holmes
- Olives, Mount of
- Olivetti
- Olivia De Havilland
- Olivier, Guillaume Antoine
- Olivier, Laurence
- Olivier Messiaen
- Olivier de Clisson
- Olivine
- Ollivier, Emile
- Olmec
- Olmert, Ehud
- Olmstead, Albert
- OloP Marco
- Olof Palme
- Olof Rudbeck
- Olof von Dalin
- Olomouc
- Olorun
- Olsen, Tillie
- Olsztyn
- Olt
- Olten
- Olunfunmilayo Ransome-Kuti
- Olusegun Obasanjo
- Olympia (Greece)
- Olympiad
- Olympian Gods
- Olympias
- Olympic Games
- Olympus
- Olynthus
- Om
- Omaha
- Omaha (ethnicity)
- Omar Al-Jawi
- Omar Bongo
- Omar Bradley
- Omar Sharif
- Omar Torrijos Herrera
- Omar bin Awad
- Omar bin Mohammad Al-Mosen
- Ombudsman
- Omega
- Omega Workshops
- Omen