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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 ... Ōbaku
  • Obama Foundation ... object language
  • Object Lessons ... Obregón, José
  • Obrenović dynasty ... occhi
  • Occhialini, Giuseppe ... Oceanarium
  • oceanarium ... Ockrent, Michael Robert
  • Ockrent, Mike ... oculocutaneous albinism
  • oculomotor nerve ... Odesskiye rasskazy
  • Odessos ... Odysseus
  • Odysseus ... Oeuvre, L’ 
  • Oeuvre, Théâtre de l’ ... Official Nationality
  • official opposition ... Ogmore Castle
  • Ognissanti Madonna ... Ohno, Apolo Anton
  • Ohno, Kazuo ... Oirat language
  • Oireachtas ... Okenfuss, Lorenz
  • OKH ... Ol’ Blue Eyes
  • Ol’chon Island ... Old City
  • Old City ... Old Marseille, Museum of
  • Old Marshal ... Old Water Tower
  • Old White, The ... Oleanna
  • oleaster ... Oliphant, Sir Mark
  • OLIPPAC ... Olmedo, José Joaquín
  • Olmedo, José Joaquín de ... Olympic torch
  • Olympic training ... Omicron Ceti
  • omicron-chlorobenzylidenemalononitrile ... On Divisions
  • On Duties ... On the Correlation of Physical Forces
  • On the Corruption of Morals in Russia ... On the Wool Track
  • On Thermonuclear War ... One Act doctrine
  • One and Three Chairs ... Onegin, Eugene
  • Oneglia ... Ontario Lacus
  • Ontario Professional Hockey League ... opaline glass
  • opalinid ... open-market operation
  • open-ocean polynya ... Ophelia
  • Ophelia ... Opole, principality of
  • Opole-Racibórz ... optical character recognition
  • optical communication ... Opus Clavicembalisticum
  • Opus de emendatione tempore ... orange
  • Orange basin ... Orbignya
  • Orbignya cohune ... Orcutt, Maureen
  • Orczy, Baroness Emmuska ... Ordo Clericorum Regularium Pauperum Matris Dei Scholarum Piarum
  • Ordo Gilbertinorum Canonicorum ... oreodont
  • Oreodonta ... organic waste
  • Organichesky Reglament ... Oriental arborvitae
  • Oriental beech ... originalism
  • Origines ... Orléans
  • Orleans ... ornithine transcarbamoylase
  • ornithine transcarbamylase deficiency ... Orphan, The
  • Orphan, The ... Orthoceras
  • orthochemical rock ... Ortstock
  • Ortygia ... Osborne, Thomas
  • Osborne, Thomas Mott ... Oslo Accords
  • Oslo and Utøya attacks of 2011 ... osseous spiral lamina
  • Osservatore Romano, L’  ... Osterbrock, Donald Edward
  • Østerdalen ... Osudy dobrého vojáka Švejka za světové války
  • Osugi Sakae ... Other Woman, The
  • Other Woman, The ... Ottawa Treaty
  • Ottawa Valley ... Ouahran
  • Oualo ... Our Ground Time Here Will Be Brief
  • Our Hearts Were Young and Gay ... Outcast of the Islands, An
  • outcaste ... Outsider, The
  • Outsider, The ... Overijssel
  • overkill hypothesis ... ovule
  • Ovulidae ... Oxalis acetosella
  • Oxalis cernua ... oxpecker
  • Oxuna ... oyster drill
  • oyster mushroom ... Ozzfest
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