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  • o ... O’Connor, Sandra Day
  • O’Connor, Sinead ... O’Neill, Hugh
  • O’Neill, Hugh, 2nd earl of Tyrone ... oak apple
  • Oak Bay ... Obama Foundation
  • Obama vs. McCain ... Object Lessons
  • object method ... Obrenović, Aleksandar
  • Obrenović, Mihailo ... occidental cat’s-eye
  • Occidental Chemical Corporation ... Oceania
  • Oceania ... OCLC Online Computer Library Center
  • Ocloo, Esther Afua ... oculomotor nucleus
  • oculopharyngeal dystrophy ... Odessus
  • Odets, Clifford ... Odysseus and Calypso
  • Odyssey ... Oeuvres de Henri Poincaré
  • Of a Fire on the Moon ... official scorer
  • Official Secrets ... Ognissanti, Church of
  • OGO ... Ohr-O’Keefe Museum of Art
  • Ohře River ... Oirot
  • Oise ... Okha
  • Okhlopkov, Nikolay Pavlovich ... Olabimtan, Afolabi
  • Olacaceae ... Old City
  • Old City Wall ... Old Master
  • Old Men at the Zoo, The ... Old Wives’ summer
  • Old Wives’ Tale, The ... oleaster family
  • olecranon ... Olisipo
  • Olissibona ... Olmert, Ehud
  • Olmi, Ermanno ... Olympic truce
  • Olympic Village ... Omīd
  • Omidyar, Pierre ... On Englishing the Bible
  • On est au coton ... On the Crown
  • On the Death of a Fair Infant Dying of a Cough ... On Translating Homer
  • On Trial ... One Big Union
  • One Day at a Time ... Oneida
  • Oneida ... Ontario, flag of
  • Ontario, Lake ... Oparin, Aleksandr
  • Oparin, Aleksandr Ivanovich ... open-reel recorder
  • open-system framework ... Opheodrys aestivus
  • Opheodrys vernalis ... Opostegidae
  • Opous ... optical frequency chain
  • optical frequency comb technique ... opus reticulatum
  • opus sectile ... orange honeysuckle
  • Orange Is the New Black ... orbital angular momentum
  • orbital element ... ordeal by fire
  • ordeal by physical test ... Ordoño IV
  • Ordos Gaoyuan ... Oresteia
  • Orestes ... organism, multicellular
  • organismic biology ... Oriental giant squirrel
  • Oriental greenfinch ... Origo mundi
  • Orihuela ... Orléans, Louis-Philippe, duc d’
  • Orléans, Louis-Philippe, duc d’ ... ornithopod
  • Ornithopoda ... Orpheus and Eurydice
  • Orpheus in the Underworld ... Orthodox Church of Ukraine
  • Orthodox Confession of the Faith, The ... Orygynale Cronykil
  • Oryol ... OSCE
  • Osceola ... Osman, Muhammad Fathi
  • Osmanabad ... ossicular chain
  • ossicular interruption ... Österreichische Volkspartei
  • Österreichischer Rundfunk ... Oswald the Lucky Rabbit
  • Oswald’s Tale ... Otis, Elisha
  • Otis, Elisha Graves ... Otto der Grosse
  • Otto der Schütz ... Oudere, Pieter Breughel de
  • Oudere, Pieter Bruegel de ... Our Lady of the Flowers
  • Our Lady of the Good Shepherd, Sisters of ... outer planet
  • Outer Seven ... oval kumquat
  • oval window ... overrun
  • overrunning clutch ... Owen, Daniel
  • Owen, David ... Oxenstierna af Södermöre, Axel, Greve
  • Oxenstierna, Bengt Gabrielsson, Greve ... oxygen and lime bottom blowing
  • oxygen availability ... Ozaki Yukio
  • Özal, Turgut ... Ozzfest
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