- o ... O’Curry, Eugene
- O’Daly, Carroll ... O’Neill, Sir Phelim
- O’Neill, Terence ... oak moss
- Oak Openings ... Obama’s Wars
- Obama, Barack ... Object to be Destroyed
- object, aesthetic ... Obrenović, Milan
- Obrenović, Miloš ... Occator
- Occhetto, Achille ... Ocean’s Deadliest
- Ocean’s Eleven ... Ockeghem, Jean de
- Ockels, Wubbo ... octothorp
- octothorpe ... Oder-Spree Canal
- Oderhaff ... odour
- odour receptor ... OEP Imaging Corporation
- oersted ... offensive huddle
- offensive realism ... Ogdensburg
- Ogdensburg Declaration ... Ohio
- Ohio ... oil plant
- oil platform ... okapi
- Okapia johnstoni ... Okovango River
- Okpame ... Olcott, Henry Steel
- Old ... Old Icelandic language
- Old Ideas ... Old Russian
- Old Russian literature ... Oldenbarnevelt, Johan van
- Oldenburg ... Olga, Saint
- Olgas ... Oliver, John E.
- Oliver, Joseph ... Olson, Charles John
- Olson, Elder ... Oman
- Oman bullhead shark ... Omo-Tana languages
- omohyoid muscle ... On Keeping Women
- On Khrushchev’s Phony Communism and Its Historical Lessons for the World ... On the Infinite Universe and Worlds
- On the Jewish Question ... on-the-job training
- Ona ... One Foot in Heaven
- One for the Money ... Onegin, Eugene
- Oneglia ... Onslow, Muriel Wheldale
- Ontake, Mount ... opacity
- opah ... open-billed stork
- open-centre system ... operations research
- Operations Research ... opioid peptide
- opioid receptor ... opstandigen, De
- Optatus of Milevis ... option
- option ... oral literature
- oral medicine ... Orapa
- OraQuick ... orchestral bells
- orchestral chimes ... Ordet
- ordinair-ozier ... Oregon Caves National Monument
- Oregon City ... organ-pipe coral
- organ-procurement organization ... Organon der rationellen Heilkunst
- organophosphate ... orifice scrubber
- Oriflamme ... Oriskany, Battle of
- Orissa ... Ormazd
- Ormazd ... orogeny
- orographic cloud ... orris root oil
- orrisroot ... orthogonal projection
- orthogonal trajectory ... Oryzomys
- Oryzomys alfaroi ... Oscines
- Osco Drug, Inc. ... Osmeroidea
- Osmeroidei ... Ostade, Isack van
- Ostade, Isak van ... ostinato
- ostium ... Oswego
- Oswego Movement ... Otidiphaps nobilis
- Otis Elevator Company ... Ottingen-Schrattenhofen faience
- Otto ... Oude Dorp
- Oude, Lievens de ... Our Lady of Good Tidings
- Our Lady of Guadalupe, Basilica of ... Outcasts, The
- Outcault, Richard Felton ... Outsider, The
- Outsider, The ... Overhead in a Balloon: Stories of Paris
- overhead intermediate proofer ... Ovis vignei
- Ovitz, Michael ... Owon
- Owono, Joseph ... oxime
- oxirane ... Oyo
- Oyo ... ozonosphere
- Ozoyong ... Ozzfest
