- O ... oak wilt
- Oakes, John Bertram ... obedience plant
- obedient plant ... obligate anaerobe
- obligate carnivore ... observational learning
- Observationes Medicae ... occlusive stroke
- occultation ... Ocean’s Eleven
- Ocean’s Eleven ... octachlor
- octadecanoic acid ... Odas para el hombre y la mujer
- Ōdate ... ODM–K
- Odnoyetazhnaya Amerika ... Odzala-Kokoua
- ’Odzer-can-ma ... Of Human Freedom
- Of Insects ... offshoot
- offshore bar ... Ogre, The
- ogre-faced spider ... Oiapoque, Rio
- OIC ... Oíti
- Ojai ... Okigbo, Christopher
- Okin, Susan Moller ... Ólafs saga Tryggvasonar
- Ólafsson, Eggert ... Old Folks at Home
- Old Forest, The ... Old Regime and the Revolution, The
- Old Régime in Canada, The ... Oldenburg, Christopher, count of
- Oldenburg, Claes ... Olid, Cristóbal de
- Olier, Jean-Jacques ... Olivétan, Pierre-Robert
- Olivetti & C. SpA ... Olt River
- Oltava ... Oman: Year In Review 2008
- Oman: Year In Review 2009 ... Omotic languages
- Ōmoto ... On Protracted War
- On Providence ... On the Road
- On the Rules of Law ... Ondangua
- Ondangwa ... O’Neal, Tatum
- O’Neale, Margaret ... “Onnazaka”
- Onnes, Heike Kamerlingh ... Oosterbaan, Benjamin Gaylord
- Oosterbaan, Bennie ... open-billed stork
- open-centre system ... Opet
- “Opfergang, Der” ... opossum shrimp
- Opostegidae ... optical isomerism
- optical lever system ... Oquendo, Antonio de
- Oquirrh Mountains ... Orange Walk
- orange-fronted parakeet ... Orbs
- orc ... order of interference
- Order of Nature, The ... “Ore’aḥ Nataʿ”
- Oreamnos americanus ... orfrey
- organ ... organogenesis
- “Organographie der Pflanzen” ... orientation
- orientation ... Orizaba
- Orizaba, Pico de ... Ormonde, James Butler, 2nd duke of
- Ormonde, Piers Butler, 8th Earl of, Earl of Ossory ... Orot ha-qodesh
- orotate ... Ortega y Gasset, José
- Ortelius, Abraham ... Ortiz, José Domingo
- Ortíz Mena, Antonio ... Osborne, George Gideon Oliver
- Osborne I ... Osireion
- Osiris ... OSRD
- Osrhoene ... osteosclerosis
- Osteostraci ... Ostyak Samoyed language
- Ostyako-Vogulsk ... Óthris, Óros
- Oti River ... otter-cat
- otterhound ... Oude, Lievens de
- Oudenaarde ... Our Lady of the Sacred Heart, Daughters of
- Our Lady of Vladimir ... outlet glacier
- Outlet, The ... overall stripping ratio
- Overami ... oviparity
- ovipositor ... Owl’s Head
- Owls to Athens ... oxido-reductase
- oxidoreductase ... Oyrat
- Oyrat language ... Ozzfest
