- O ... O’Connor, William
- O’Curry, Eugene ... O’Neill, John
- O’Neill, Michelle ... ÖAIG
- oak ... Obadiah of Bertinoro
- Obadiah, Book of ... obiter dictum
- Obito ... obole
- Obolellida ... Obžčij Syrt
- Oc Eo ... Ocean Drilling Project
- ocean ecosystem ... Ochoa, Lorena
- Ochoa, Severo ... October plant
- October Revolution ... Ode of the Theotokos
- Ode on a Distant Prospect of Eton College ... Odobenus rosmarus
- Odobenus rosmarus divergens ... oedipal stage
- Oedipe ... Of the Nature and Qualification of Religion in Reference to Civil Society
- Of the Nature of Virtue ... offshore bar
- offshore drilling ... Ogowe
- Ogowe River ... Ohno Taiichi
- Ohno, Apolo ... oinochoe
- ointment ... Okeh Records
- Okehampton ... Ōkuninushi
- Ōkuninushi no Mikoto ... Old Bern
- Old Bet ... Old Lithuanian language
- Old Logic ... old squaw duck
- Old State Capitol ... Oldham, John
- Oldham, Richard Dixon ... oligodendrocyte
- oligodendroglioma ... Olivetti, Adriano
- Olivetti, Camillo ... Olson, Mark
- Olson, Nancy ... Omagh bombing
- Omagua ... omnibus hearts
- omnidirectional antenna ... On Her Majesty’s Secret Service
- On Heroes and Tombs ... On the Eve
- On the Fifth of November ... On War
- On What Matters ... One Beat
- One Belt, One Road ... one-dimensional array
- One-Dimensional Criticism ... Only Murders in the Building
- Only Son, The ... oolong
- oomblik in die wind, ’N ... open primary
- open question argument ... opéra-comique
- Opéra-Comique ... Ophioderma brevispina
- Ophiodon elongatus ... Opoponax, The
- Oporto ... optical disc
- optical double resonance ... Opus
- opus alexandrinum ... Orange
- Orange ... Orbán Viktor
- Orbán, Viktor ... Orchis mascula
- Orchis militaris ... ordinary portland cement
- ordinary ray ... Oregonian, The
- Oreiller, Henri ... organic geochemistry
- Organic Law ... orgone box
- Orgone Energy Accumulator ... Origin of Ideas, The
- origin of life ... Orkhan
- Orkhon inscriptions ... Ormond Beach
- Ormonde ... Orontes River
- oronymy ... Orsini, Marina
- Orsini, Pietro Francesco Vincenzo Maria ... orthonectid
- Orthonectida ... Os
- OS ... Oseberg ship
- Osee ... osmium
- osmium-186 ... Ostariophysi
- Ostaynitsa ... Ostland
- Østlandet ... Oswego Plan
- Oswego tea ... Otididae
- Otidiphaps nobilis ... otterhound
- Otters, Algae, and Plants, Oh My ... oud
- oud ... Our Kind of Traitor
- Our Kind of Traitor ... Outaouais River
- Outardes River ... Outremer
- Outremeuse, Jean d’ ... overcrowding
- overdenture ... Oviedo
- Oviedo, Fernández de ... Owings, Nathaniel
- owl ... Oxford-AstraZeneca COVID-19 vaccine
- Oxfordian Stage ... oxytocin
- Oxytropis lambertii ... Ozero Zaysan
- Ozersk disaster ... Ozzman
