- O ... O’Daly, Carroll
- O’Daly, Demetrio ... O’Neill, Tip
- O’Neill, Turlough Luineach, Earl of Clanconnell ... Oak Ridge Cemetery
- Oak Ridge National Laboratory ... Obama, Mount
- Obama/Biden ... object-relations theory
- Objections to Sex and Violence ... obscenity
- obsceno pajaro de la noche, El ... Occidental, Cordillera
- Occidental, Cordillera ... Oceanarium
- Oceania ... OCOG
- Oconee ... Oculi, Okello
- oculist ... Odessa
- Odessa ... Odul
- Odum, Howard W. ... oestrous cycle
- oestrus ... Office of Management and Budget
- Office of Strategic Services ... Ogilvy, David
- Ogilvy, David Mackenzie ... Ohio Statehouse
- Ohio University ... oil window
- Oil! ... OKB-1
- OKB-155 ... Ōkuma Shigenobu
- Okumura Masanobu ... Old Babylonian Empire
- Old Bachelour, The ... Old Line State
- Old Lithuanian language ... old squaw duck
- Old State Capitol ... Oldman River
- Oldman, Gary ... oligomictic orthoconglomerate
- oligonucleotide ... Olivier, Laurence, Baron Olivier of Brighton
- Olivier, René ... Olweus, Dan
- Olybrius ... Omega Man, The
- Omega Workshops ... On a Clear Day You Can See Forever
- On a Clear Day You Can See Forever ... On Shakespeare
- on side ... On the Road
- On the Road ... Oncken, Johann Gerhardt
- oncogene ... One of Ours
- One of the Boys ... Onitsha
- Onitsha ... Onychoteuthis
- Onygenales ... Open City
- open classroom ... opera buffa
- Opera Company of Boston ... Ophioglossum
- Ophioglossum malvie ... Oppen, George
- Oppenheim, E. Phillips ... optical interferometer
- optical invariant ... opus tessellatum
- opus testaceum ... orange jessamine
- orange juice ... orbital mirror and sunshade
- orbital parameter ... ordeal by water
- Ordeal of Mark Twain, The ... Ordoño IV
- Ordos Gaoyuan ... Orešković, Tihomir
- Oresme, Nicholas ... Organisation Météoroligique Mondiale
- Organisation Mondiale de la Santé ... Oriental earthworm
- Oriental fruit fly ... Origins of the Islamic State, The
- Origins of the New South 1877–1913 ... Orléans, Charles, duc d’
- Orléans, Council of ... ornithischian
- ornithochory ... Orphan, The
- Orphan, The ... ortho-hydroxybenzoic acid
- ortho-sulfobenzoic acid imide ... Orton, Edward, Jr.
- Orton, Joe ... Osborne, John James
- Osborne, Sarah ... Oskaloosa
- Öskemen ... Oss
- Ossa ... osteosarcoma
- osteosclerosis ... Ostwald, Carl Wilhelm Wolfgang
- Ostwald, Friedrich Wilhelm ... Other Boleyn Girl, The
- Other Campaign, The ... otro rostro del peronísmo, El
- Otrochestvo ... Ou River
- Ou River ... Our American Cousin
- Our Bodies Ourselves ... Out of My System: Psychoanalysis, Ideology, and Critical Method
- Out of Order: Stories from the History of the Supreme Court ... Outlook, The
- Outokumpu ... Overbeck, Franz
- Overbeck, Fritz ... Ovibos moschatus
- Ovid ... OWG
- OWI ... Oxford-AstraZeneca COVID-19 vaccine
- Oxfordian Stage ... oxytetracycline
- oxytocin ... Ozette site
- Ozhog ... Ozzfest