| Osteoclast | Osteoclasts | Osteocyte |
| Osteogenesis imperfecta | Osteoglossidae | Osteoglossiformes |
| Osteoglossinae | Osteoglossoidei | Osteoglossomorpha |
| Osteoglossum | Osteoglossum bicirrhosum | Osteoglossum ferreirai |
| Osteoimmunology | Osteolaemus tetraspis | Osteolipochondroma |
| Osteology | Osteolysis | Osteomalacia |
| Osteomyelitis | Osteopathic medicine | Osteopathic medicine in the United States |
| Osteopetrosis | Osteophyte | Osteopilus septentrionalis |
| Osteoporosis | Osteosarcoma | Osteotomy |
| Oster Conspiracy | Osterdock, Iowa | Osterholz-Scharmbeck |
| Osterode am Harz | Ostium primum atrial septal defect | Ostomy pouching system |
| Ostraciidae | Ostracion | Ostracod |
| Ostracoda | Ostracoderm | Ostracodermi |
| Ostracon | Ostrander, Minnesota | Ostrander, Ohio |
| Ostrea | Ostreville | Ostrich |
| Ostrinia nubilalis | Ostrogoths | Ostrya carpinifolia |
| Ostwind | OSU Open Source Lab | Osuosl |
| Osvaldo Ardiles | Osvaldo Benavides | Osvaldo Ríos |
| Osvaldo Velloso de Barros | Osvaldo Vieira International Airport | Oswald Avery |
| Oswald Ducrot | Oswald Spengler | Oswald von Wolkenstein |
| Oswaldo Alanís | Oswaldo Guimarães | Oswaldo Sánchez |
| Oswaldo Vizcarrondo | Oswegatchie, New York | Oswego (town), New York |
| Oswego Movement | Oswego, Kansas | Oswego, New York |
| OSx86 | Osyka, Mississippi | Osyris quadripartita |
| Ota Benga | Otacon | Otago |
| Otaku | Otalgia | Otaq Sara, Amol |
| Otaria flavescens | Otariidae | Otariinae |
| Otedoh | Otego (village), New York | Otero (Spain) |
| Otfried Preußler | Othello | Othello, Washington |
| Other | Other universes | Others (Lost) |
| Othman Almulla | Othman Benjelloun | Othman Jerandi |
| Othmar Spann | Otho | Otho, Iowa |
| Otic ganglion | Otididae | Otilonium bromide |
| Otis Elevator Company | Otis tarda | Otis, Colorado |
| Otis, Kansas | Otis, Maine | Otis, Massachusetts |
| Otisco Township, Michigan | Otisco, New York | Otisfield, Maine |
| Otisville, Michigan | Otisville, New York | Otitis |
| Otitis externa | Otitis media | Otl Aicher |
| Otman Bakkal | Oto, Iowa | Otoacoustic emission |
| Otocephaly | Otocolobus manul | Otocyon megalotis |
| Otoe, Nebraska | Otokar Arma | Otomat |
| Otorhinolaryngology | Otoscope | Otospermophilus variegatus |
| Otostegia | Otostegia ellenbeckii | Otostegia ericoidea |
| Otostegia erlangeri | OTR-21 Tochka | OTRS |
| OTS 44 | Otsego Lake Township, Michigan | Otsego Township, Michigan |
| Otsego, Michigan | Otsego, Minnesota | Otsego, New York |
| Otsego, Wisconsin | Otselic, New York | Ottawa |
| Ottawa Hills, Ohio | Ottawa Treaty | Ottawa, Illinois |
| Ottawa, Kansas | Ottawa, Wisconsin | Ottelia |
| Ottelia acuminata | Ottelia alismoides | Ottelia balansae |
| Ottelia brachyphylla | Ottelia brasiliensis | Ottelia brevifolia |
| Ottelia cordata | Ottelia cylindrica | Ottelia emersa |
| Ottelia exserta | Ottelia fischeri | Ottelia japonica |
| Ottelia kunenensis | Ottelia lisowskii | Ottelia luapulana |
| Otter | Otter Creek, Dunn County, Wisconsin | Otter Creek, Eau Claire County, Wisconsin |
| Otter Lake, Michigan | Otter Lake, Quebec | Otterbein, Indiana |
| Ottertail, Minnesota | Otterville, Illinois | Ottilie Wildermuth |
| Ottmar Hitzfeld | Otto | Otto Addo |
| Otto Andersson (footballer) | Otto Bardenhewer | Otto Barić |
| Otto Bühler | Otto Braun | Otto Braun (Li De) |
| Otto cycle | Otto Devrient | Otto Diels |
| Otto Erich Hartleben | Otto Ernst Remer | Otto Flake |
| Otto Fredrik Tullberg | Otto Fritz Meyerhof | Otto Gildemeister |
| Otto Grotewohl | Otto Hahn | Otto Heinrich Warburg |
| Otto Julius Bierbaum | Otto Karl Berg | Otto Kuntze |
| Otto Lilienthal | Otto Loewi | Otto Manninen |
| Otto Nerz | Otto Neurath | Otto of Freising |
| Otto Peretzl | Otto Pfister | Otto Preminger |
| Otto Pretzl | Otto Rank | Otto Rehhagel |
| Otto Schindewolf | Otto Schmidt | Otto Siffling |
| Otto Sirgo | Otto Stapf | Otto Stern |
| Otto Township, Michigan | Otto von Bismarck | Otto von Guericke |
| Otto von Kotzebue | Otto von Münchhausen | Otto Wallach |
| Otto Weddigen | Otto Wilhelm Sonder | Otto Wille Kuusinen |
| Otto, New York | Ottobiano | Ottoman architecture |
| Ottoman Aviation Squadrons | Ottoman Cyprus | Ottoman Dynasty |
| Ottoman Empire | Ottoman fleet organisation during the Greco-Turkish War (1897) | Ottoman invasion of Otranto |
| Ottoman lira | Ottoman military band | Ottoman return of Mecca 1813 |
| Ottoman ship Mahmudiye | Ottoman wars in Europe | Ottoman–German alliance |
| Ottoman–Mamluk War (1516–17) | Ottoman–Persian War (1722–27) | Ottoman–Persian War (1730–35) |
| Ottoman–Persian War (1743–46) | Ottoman–Persian War (1775–76) | Ottoman–Persian War (1821–23) |
| Ottoman–Persian Wars | Ottoman–Portuguese conflicts | Ottoman–Safavid War (1603–18) |
| Ottoman–Safavid War (1623–39) | Ottoman–Wahhabi War | Ottonian dynasty |
| Ottosen, Iowa | Ottoville, Ohio | Ottrelite |
| Ottumwa, Iowa | Otus | Otus brucei |
| Otus scops | Otv | OTV |
| Otway, Ohio | Ouachita Parish, Louisiana | Ouad Essafa |
| Ouadi Doum air raid | Ouagadougou | Ouagadougou Airport |
| Oualqadi | Ouaouizeght | Ouargla |
| Ouarzazate Airport | Ouarzazate Province | Ouday Raad |
| Oudenone | Oudjat | Oued Amlil |
| Oued Chorfa | Oued Ed-Dahab-Lagouira | Oued El Makhazine |
| Oued Fodda | Oued Irara–Krim Belkacem Airport | Oued L'Bour |
| Ouida | Ouided Kilani | Ouija |
| Ouija (2014 film) | Ouizeght | Oujé-Bougoumou, Quebec |
| Oujda | Oujda Group | Oulad Aissa |
| Oulad Ali Youssef | Oulad Ayyad | Oulad Ben Hammadi |
| Oulad Berhil | Oulad Bourima | Oulad Chrif |
| Oulad Daoud | Oulad Gnaou | Oulad Hcine |
| Oulad M'Rah | Oulad Moumna | Oulad Nouel |
