- o ... O’Curry, Eugene
- O’Daly, Carroll ... O’Neill, Paul
- O’Neill, Paul ... oak apple
- Oak Bay ... Obaid, Thoraya Ahmed
- Ōbaku ... object
- object case ... Obradović, Dositej
- Obras completas ... Ocampo, Silvina
- Ocampo, Victoria ... ocean marine insurance
- ocean of milk, churning of the ... ochronosis
- ochronotic arthropathy ... Octomeles sumatrana
- octopi ... Ode: Intimations of Immortality
- Odebrecht ... Odonotornith
- Odontaspididae ... Oedogonium
- OEEC ... Off-Off-Broadway
- off-price retailer ... Ogasawara-guntō
- Ogata Ichinojō ... Oh Doctor
- Oh Eun-Sun ... oil burner
- oil cake ... Ojing-eo Geim
- Ōjō Yōshū ... Okinawa Trough
- Okinawa, Battle of ... Ólafs saga helga
- Ólafs saga Tryggvasonar ... Old Dreadnought
- Old Economy Village ... Old Norse language
- Old North Arabian language ... Old World deer
- Old World flamingo ... Olekma River
- Oleksandrivsk ... Oliva, Rodrigo Calderón, conde de
- Oliva, Tony ... Olmstead v. L.C.
- Olmsted Act ... Olympic torch
- Olympic truce ... Omer Seyfettin
- Omer-paša Latas ... On Call: A Doctor’s Journey in Public Service
- On Care for Our Common Home ... On the Beach
- On the Beach ... On the Special Laws
- On the Spectacles ... Oncorhynchus tshawytscha
- Oncosperma ... One Thousand Guineas
- One Ton Cup ... Onizuka, Ellison
- Onkelos, Targum of ... Onygenales
- onyx ... open classroom
- open cluster ... Opera and Drama
- Opera aperta ... Ophichthidae
- Ophichthus ophis ... Opobo
- Opoku Ware ... optical axis
- optical bleach ... Opuntia
- Opuntia basilaris ... Orang Jawa
- Orang Melayu ... orb
- orb weaver ... orchil
- Orchis ... Ordinary of the Mass
- Ordinary People ... Oregon, flag of
- Oregon, University of ... organic geochemistry
- Organic Law ... orgone box
- Orgone Energy Accumulator ... origin of life
- Origin of Races, The ... Orkhon Turk
- Orkin, Ruth ... Ormonde, earls and dukes of
- Ormonde, James Butler, 12th earl and 1st duke of ... Oroonoko
- Oroonoko ... Orsini, Valentino
- Orsino ... orthophosphoric acid
- orthophosphorous acid ... Os simples
- Os Sonetos Completos ... osetrova caviar
- Osgood’s rat ... osmotic regulation
- osmotrophic nutrition ... ostensive definition
- Ostenso, Martha ... ostracod
- Ostracoda ... Otago, University of
- Otaheite ... Otito Koro
- Otiyyot de Avraham Avinu ... otto of rose
- Otto Perl Alliance ... Oued, el-
- Oueddei, Goukouni ... Our Lady of the Sacred Heart, Daughters of
- Our Lady of Tortosa, Cathedral of ... Outer Hebrides
- outer Helmholtz plane ... outwash plain
- Ouvéa Island ... overland flow
- Overland Mail Company ... Ovulidae
- ovum ... oxalic acid
- Oxalidales ... oxodecenoic acid
- α-oxoglutarate ... Øystein I Magnusson
- oyster ... Ozzfest
