- o ... O’Daly, Carroll
- O’Daly, Demetrio ... O’Neill, Terence
- O’Neill, Thomas P., Jr. ... Oak Park
- Oak Ridge ... Obama, Barack Hussein, II
- Obama, Barack Hussein, Sr. ... object-oriented database
- object-oriented language ... Obri
- Obrona żarliwoṡci ... Occhialini, Giuseppe
- occidental cat’s-eye ... Ocean’s Twelve
- ocean-atmosphere interaction ... Ockham’s razor
- Ockham, William ... ocular
- ocular accommodation ... Odes on Several Descriptive and Allegoric Subjects
- Odesa ... Odría Amoretti, Manuel Arturo
- Odría, Manuel A. ... oesophageal voice
- oesophagus ... office
- office building ... ogham alphabet
- ogham writing ... Ohio Idea
- Ohio Land Company ... oil shale
- oil sketch ... Ōkawa Shūmei
- Okaya ... oktōēchos
- Oktogon ... Old and The Young, The
- Old Arbat ... Old Jew and a Boy
- Old Jules ... Old School
- Old School Presbyterian Church ... Oldest Living Confederate Widow Tells All
- oldfield birch ... Oligocene Epoch
- Oligochaeta ... Olivétan, Pierre-Robert
- Olivetti & C. SpA ... Olsztyn
- Olt ... Omar, Mullah
- omasum ... Omphalea
- Omphalea diandra ... On Our Selection
- On Our Selection ... On the Nature of Animals
- On the Nature of the Gods ... Once Around
- Once Around ... One IBM Plaza
- One in a Million ... Ong Bun
- Ong Lo ... ontological reductionism
- ontologism ... OPD
- OPDA ... open-wire pair
- open-wire transmission line ... Ophelia
- Ophelia ... Oplegnathidae
- OPM ... optical art
- optical axis ... Opuntia megacantha
- Opuntia vulgaris ... orange
- Orange ... Orbe River
- Orbea, Fernando de ... Orchis militaris
- Orchis morio ... Ordinary Woman, An
- Ordinatio ... Orekhovo-Zuyevo
- Orël ... organic phosphorus compound
- organic pigment ... oribatid mite
- Oribatida ... original acquisition
- Original African Hebrew Israelite Nation of Jerusalem, The ... Orlando
- Orlando ... ornament
- ornamental ... Orot ha-qodesh
- orotate ... Ortalis
- Ortalis vetula ... orthorhombic sulfur
- orthorhombic system ... Ōsaka Asahi
- Ōsaka Bay ... Oshitelu, Josiah Olunowo
- Oshkosh ... Osnovy Khimii
- Osnovyanenko, Hryhorii Kvitka ... Osteodontokeratic tool industry
- osteodystrophy fibrosa ... Ostrog, Michael
- Ostrogorsky, Moisey Yakovlevich ... OTEC
- Otechestvenofront ... Otolemur crassicaudatus
- otolith ... Ottoman court carpet
- Ottoman Empire ... Ould Salek, Mustapha
- Ouled Naïl ... Our Town
- Our Ukraine ... outflow channel
- outgassing ... ovarian bursa
- ovarian cancer ... Overseas Needlework and Crafts Project
- Overseas Surveys Directorate ... Owen, Grace
- Owen, John ... Oxenstierna, Johan Gabriel
- Oxenstierna, Jöns Bengtsson ... oxygen acid
- oxygen and lime bottom blowing ... Oz, Mehmet Cengiz
- Ozaki Kōyō ... Ozzfest