Operation Weserübung – Oral Torah edit
Operation Weserübung · Operation Winter Storm · Operation Wrath of God · Operational amplifier · Operations management · Operations research · Operator · Operetta · Operon · Opeth · Ophiuchus · Ophthalmology · Max Ophüls · Opiliones · Opinion · Opinion poll · Opioid · Opisthobranchia · Opium · Second Opium War · Opium Wars · Opium poppy · Opole · Opole Voivodeship · Opossum · Virginia Opossum · J. Robert Oppenheimer · Opportunity rover · Opportunity cost · The Oprah Winfrey Show · Optic nerve · Optical character recognition · Optical fiber · Optical illusion · Optical microscope · Optics · Optimism · Optimization (mathematics) · Optimus Prime (Transformers) · Option (finance) · Optometry · Opuntia · Opus Dei · Terry O'Quinn · Oracle Corporation · Oracle Database · Oradour-sur-Glane · Oral cancer · Oral sex · Oral Torah
Oral and maxillofacial surgery – Order of the Golden Fleece edit
Oral and maxillofacial surgery · Oral rehydration therapy · Oran · Orange (colour) · Orange (fruit) · The Orange Box · Orange County, California · Orange County, New York · Orange Free State · Orange Institution · Orange juice · Orange Prize for Fiction · Orange Revolution · Orange River · Orange, California · Orange, Vaucluse · House of Orange-Nassau · Orangutan · Oranjestad, Aruba · The Orb · Orb-weaver spider · Leonard Orban · Roy Orbison · Orbit · Orbit of the Moon · Orbital (band) · Orbital elements · Orbital mechanics · Orbital period · Orbital resonance · Orbital eccentricity · Orc (Middle-earth) · Orchard · Orchestral Manoeuvres in the Dark · Orchidaceae · 90482 Orcus · Order (biology) · Order of Aviz · Order of Canada · Order of Friars Minor Capuchin · Order of Saint Benedict · Order of the British Empire · Order of the Garter · Order of the Phoenix (organisation) · Order of the Thistle · Order of Victory · Order of Australia · Order of magnitude · Order of the Bath · Order of the Golden Fleece
Order of the Red Banner – Organization of American States edit
Order of the Red Banner · Order of the White Eagle (Poland) · Order theory · Ordered pair · Ordinal number · Ordinary differential equation · Ordnance QF 6 pounder · Ordnance Survey · Ordo Templi Orientis · Ordovician · Ordu Province · Ore · Örebro · Oregano · Oregon · Oregon State Capitol · Oregon State University · Oregon Trail · Oregon Treaty · University of Oregon · Bill O'Reilly (cricketer) · Bill O'Reilly (political commentator) · Orenburg · Orenburg Oblast · Oreo · Øresund · Øresund Bridge · Orfeo ed Euridice · Carl Orff · Horace Walpole, 4th Earl of Orford · Organ (music) · Organ donation · Princess Leia Organa · Organelle · Organic architecture · Organic chemistry · Organic compound · Organic farming · Organic food · Organic synthesis · Organic acid · Organisation for Economic Co-operation and Development · Organisation internationale de la Francophonie · Organisation of African Unity · Organisation of the Islamic Conference · Organism · Organization · Organization for Security and Co-operation in Europe · Organization of Ukrainian Nationalists · Organization of American States
Organizational culture – Oromia Region edit
Organizational culture · Organizational structure · Organized crime · Organochloride · Organometallic chemistry · Organophosphate · Orgasm · Orhan I · Naser Orić · Oricon · Oriel College, Oxford · Orient Express · Oriental Mindoro · Oriental Orthodoxy · Orienteering · Origami · Origen · Origin of birds · Origin of language · On the Origin of Species · Origin of the domestic dog · Origin of the Romanians · Original sin · Original video animation · Origins of the American Civil War · Orinoco · Oriole · Orion (spacecraft) · Orion (constellation) · Orion (mythology) · Orion Nebula · Orissa · Oriya language · Pico de Orizaba · Orkney · Orkut · Orlando Magic · Orlando, Florida · Orléans · Orlov · Eugene Ormandy · Ornament (architecture) · Ornamental plant · Orne · Ornithology · Örnsköldsvik · Leo Ornstein · Orochimaru (Naruto) · Orogeny · Oromia Region
Oromo language – Oshawa edit
Oromo language · Oromo people · Orphan · Orphanage · The Orphanage (2007 film) · Orpheus · Bobby Orr · Musée d'Orsay · Hans Christian Ørsted · José Ortega y Gasset · Daniel Ortega · Orthodox Autocephalous Church of Albania · Orthodox Church in America · Orthodox Judaism · Orthogonal group · Orthogonal matrix · Orthogonal polynomials · Orthogonality · Orthography · Orthomyxoviridae · Orthoptera · Orthorhombic crystal system · Orthostatic hypotension · Beth Orton · Joe Orton · Randy Orton · Orūzgān Province · Oruç Reis · Orvieto · George Orwell · Oryol · Oryol Oblast · Osaka · Osaka Castle · Osaka Prefecture · Osaka University · Osama bin Laden · CA Osasuna · George Osborne · John Osborne · Kelly Osbourne · Ozzy Osbourne · Sharon Osbourne · Oscar (fish) · Oscar I of Sweden · Oscar II of Sweden · Oscillation · Oscilloscope · Oseltamivir · Oshawa
John O'Shea – Otaku edit
John O'Shea · Osho (Bhagwan Shree Rajneesh) · OSI model · Osijek · Osiris · William Osler · Oslo · Oslo Accords · Oslo Airport, Gardermoen · Oslo Metro · Osman I · Osmaniye Province · Sergio Osmeña · Emily Osment · Haley Joel Osment · Osmium · Osmosis · Osmotic pressure · Osnabrück · Osprey · Ossetians · Ossetic language · Carl von Ossietzky · Ostankino Tower · Osteichthyes · Ostend · Ostend Manifesto · Osteoarthritis · Osteochondritis dissecans · Osteoclast · Osteogenesis imperfecta · Osteoporosis · Osteosarcoma · Österreichischer Rundfunk · Östersund · Østfold · Ostracod · Ostrava · Ostrich · Ostrogoths · Elinor Ostrom · Wilhelm Ostwald · Ronnie O'Sullivan · Oswald of Northumbria · Lee Harvey Oswald · Oswego County, New York · Oświęcim · Oswiu of Northumbria · Ōta, Tokyo · Otaku
Otello – Outkast edit
Otello · Othello · The Other Woman · Otho · Otitis externa · Otitis media · Otley · Oto-Manguean languages · Otolaryngology · Peter O'Toole · Otsego County, New York · Ottawa · Ottawa Senators (original) · Ottawa Macdonald-Cartier International Airport · Ottawa River · Ottawa Senators · University of Ottawa · Otter · Otto I, Holy Roman Emperor · Otto II, Holy Roman Emperor · Otto III, Holy Roman Emperor · Otto IV, Holy Roman Emperor · Otto of Greece · Miranda Otto · Ottokar II of Bohemia · Ottoman Dynasty · Ottoman Empire · Ottoman Greece · Ottoman Turkish language · Ötzi the Iceman · Ouagadougou · Oud · Nicolas Oudinot · Maryse Ouellet · Ouija · Oulu · Ounce · Our Airline · Our Gang · Our Lady of Fátima · Our Lady of Lourdes · Our Lady Peace · Our Ukraine–People's Self-Defense Bloc · Out of Africa · Outback · Outer Hebrides · The Outer Limits (1995 TV series) · Outer space · Outfielder · Outkast
Outlaw – University of Oxford edit
Outlaw · Outrageous · Outsourcing · Ouyang Xiu · Oval Office · Ovarian cancer · Ovarian cyst · Ovary · Alexander Ovechkin · Oven · Overfishing · Overhead camshaft · Overhead lines · Overhead valve · Overijssel · Operation Overlord · Overpopulation · Overseas Chinese · Overseas Filipino · Overtone · 1812 Overture · Overweight · Ovid · Oviedo · Ovum · Owain Glyndŵr · Owen submachine gun · Clive Owen · Michael Owen · Richard Owen · Robert Owen · Wilfred Owen · Buck Owens · Jesse Owens · Terrell Owens · Owensboro, Kentucky · Owl · Owl City · Ownership · Ox · Oxalic acid · Oxazepam · Oxbow lake · Axel Oxenstierna · Oxford · Oxford English Dictionary · Oxford Street · Oxford United F.C. · Oxford University Press · University of Oxford
Oxfordshire – Ozzfest edit
Oxfordshire · Oxidation number · Oxidation state · Oxidative phosphorylation · Oxide · Oxidizing agent · Oxnard, California · Oxy-fuel welding and cutting · Oxycodone · Oxygen · Oxygen toxicity · Oxymoron · Oxyrhynchus · Oxytocin · Oyster · Oyster card · Amos Oz · Frank Oz · Andranik Toros Ozanian · Maria Ozawa · Mesut Özil · Ozone · Ozone depletion · Ozone layer · Yasujirō Ozu · Ozzfest