| Ostraka | Ostracon |
| Ostrava | Ostrava |
| ostrica | oyster |
| ostricoltura | oyster farming |
| Ostrinia nubilalis | European Corn Borer |
| ostro | south |
| Ostrobotnia | Ostrobothnia |
| Ostrobotnia centrale | Central Ostrobothnia |
| Ostrobotnia meridionale | Southern Ostrobothnia |
| Ostrobotnia settentrionale | Northern Ostrobothnia |
| Ostrogoti | Ostrogoths |
| Ostrogožsk | Ostrogozhsk |
| Ostrov | Ostrov |
| Ostrovnoj | Ostrovnoy |
| Ostrowiec Świętokrzyski | Ostrowiec Świętokrzyski |
| Ostrołęka | Ostrołęka |
| ostruire | stop |
| ostruito | clogged |
| ostruzione | obstruction |
| ostruzionismo | obstructionism |
| ostruzionista | obstructive |
| Ostróda | Ostróda |
| Ostrów Mazowiecka | Ostrów Mazowiecka |
| Ostrów Tumski | Ostrów Tumski in Wrocław |
| Ostrów Wielkopolski | Ostrów Wielkopolski |
| Ostsiedlung | Ostsiedlung |
| Osumi | Ōsumi Province |
| Osun | Osun State |
| Osvaldo | Oswald of Northumbria |
| Osvaldo Ardiles | Osvaldo Ardiles |
| Osvaldo Cavandoli | Osvaldo Cavandoli |
| Osvaldo Dorticós Torrado | Osvaldo Dorticós Torrado |
| Oswald di Worcester | Oswald of Worcester |
| Oswald Gracias | Oswald Gracias |
| Oswald Mosley | Oswald Mosley |
| Oswald Pohl | Oswald Pohl |
| Oswald Spengler | Oswald Spengler |
| Oswald Theodore Avery | Oswald Avery |
| Oswald West State Park | Oswald West State Park |
| Oswiecim | Oswiecim |
| Oswine di Deira | Oswine of Deira |
| Oswiu di Northumbria | Oswiu of Northumbria |
| OT | OT |
| OTA | OTA |
| Otaci | Otaci |
| Otago | Otago |
| otalgia | earache |
| Otar Iosseliani | Otar Iosseliani |
| otarda | bustard |
| otaria | sea lion |
| otaria orsina | fur seal |
| Otariidae | Eared seal |
| Otavite | Otavite |
| Otelfingen | Otelfingen |
| Otello | Othello |
| Otep | OTEP |
| Otepää | Otepää |
| OTF | OTF |
| Otfried Preussler | Otfried Preußler |
| Othello | Reversi |
| Other Voices | Other Voices |
| Othmarsingen | Othmarsingen |
| Othmàn ibn Affàn | Uthman |
| Othniel Charles Marsh | Othniel Charles Marsh |
| Oti | Oti River |
| Otira | Otira |
| Otis Elevator Company | Otis Elevator Company |
| Otis Redding | Otis Redding |
| Otis tarda | Great Bustard |
| Otite | Ear infection |
| Otite media | Ear infection |
| Otmar Freiherr von Verschuer | Otmar Freiherr von Verschuer |
| OTO | Oto |
| otoiatra | ear specialist |
| Otolite | Otolith |
| Otone | Otho |
| Otowi | Otowi |
| Otočac | Otočac |
| Otranto | Otranto |
| Otrera | Otrera |
| otricolare | utricular |
| OTRS | OTRS |
| Ottaedro troncato | Truncated octahedron |
| ottagonale | octagonal |
| ottagono | octagon |
