- O ... O’Connor, William
- O’Curry, Eugene ... O’Neill, Michelle
- O’Neill, Owen Roe ... oak and beech family
- Oak and Ivy ... Obafemi Awolowo University
- Obaid, Thoraya ... OBJ
- object ... Obote, Apollo Milton
- Obote, Milton ... Öcalan, Abdullah
- Ocalenie ... ocean floor
- Ocean Greyness ... Ochotona pusilla
- Ochotonidae ... Octodon
- Octodon bridgesi ... Ode to Psyche
- Ode to the Confederate Dead ... Odon de Bayeux
- Odon de Châtillon-sur-Marne ... Oedipus II
- Oedipus on the Road ... Ofenrohr
- off road racing ... Ogadai
- Ogaden ... Oğuz
- Oğuz Turkic languages ... Oidheadh Chloinne Uisneach
- Oidipous Tyrannus ... Oistrakh, Igor
- Oistrakh, Igor Davidovich ... Oki Islands
- Oki-Daitō Island ... Olaf Guthfrithson
- Olaf I Tryggvason ... Old Coastal Plain
- Old Colony Building ... Old Man’s Letters to a Young Prince
- Old Man’s Love Story, The ... Old Town
- Old Town ... Ole Miss
- Olea europaea ... Olimpico, Teatro
- Olinda ... Oll Synnwyr Pen Kembero Ygyd
- ollada ... Olympian Zeus, temple of
- Olympias ... omasum
- Omayyad dynasty ... Omphalea megacarpa
- Omphalea triandra ... On Our Selection
- On Overgrown Paths ... On the Nature of Animals
- On the Nature of the Gods ... Once Again
- Once and Future King, The ... One Hundred and One Dalmatians
- One Hundred and One Nights ... Oneirocritica
- oneiromancy ... Ontario
- Ontario ... opah
- opal ... open-centre system
- open-couple dance ... operational experiment
- operational fatigue ... Öpik, Ernst Julius
- Opiliaceae ... Opposition
- opposition surge ... optics
- optics, X-ray ... Oracle Systems Corporation
- Oracular Chinese language ... orange-tip butterfly
- Orangeburg ... orc
- orca ... Order No. 1
- Order of American Knights ... Ordzhonikidze
- Ordzhonikidze, Grigory Konstantinovich ... Orfeo
- Orfeo ed Euridice ... organization development
- Organization of American States ... Oriental Lowestoft
- Oriental Orthodox church ... Origo mundi
- Orihuela ... Orléans, kingdom of
- Orléans, Louis I, duc d’ ... Ornithology
- ornithology ... Orphans of the Storm
- Orphée ... Orthoceras
- orthochemical rock ... Ortstock
- Ortygia ... Osborne, Thomas
- Osborne, Thomas Mott ... Oskaloosa
- Öskemen ... OSS
- Oss ... osteopetrosis
- osteophyte ... Ostrya virginiana
- Ostsee ... Othello
- Othello ... Otra Margarita
- Otrante, Joseph Fouché, duc d’ ... Otus trichopsis
- Otway, Terence ... Oun Kham
- OUN-B ... Ouse, River
- Ouse, River ... outlawry of war movement
- Outlaws of the Marsh ... ovary
- ovatio ... overseas trade
- Overseas, Council of ... Owen, Goronwy
- Owen, Grace ... Oxenstierna, Axel, Greve
- Oxenstierna, Bengt Gabrielsson, Greve ... Oxyfast
- oxygen ... Oz, Frank
- Oz, Mehmet ... Ozzman
