rumänische Ortsnamen
| Oarba de Mures |
| Oarda de Jos |
| Oarda de Sus |
| Oarzina |
| Obirsa |
| Obranesti |
| Obreja |
| Ocisor |
| Ociu |
| Ocland |
| Oclos |
| Ocna Dejului |
| Ocna de Jos |
| Ocna de Sus |
| Ocna Mures |
| Ocna Sibiului |
| Ocnisoara |
| Ocoale |
| Ocolis |
| Ocolisel |
| Ocolisu Mare |
| Ocolisul Mic |
| Odorheiu Secuiesc |
| Odrihei |
| Odverem |
| Ogra |
| Ohaba (bei Fagaras) |
| Ohaba (bei Deva) |
| Ohaba de sub Piatra |
| Ohaba Ponor |
| Ohaba Sibisel |
| Ohaba Steiului |
| Oiejdea |
| Oituz |
| Ojdula |
| Olariu |
| Olteni |
| Oltet |
| Oprea-Cirtisoara |
| Oprisani |
| Oprisesti |
| Oraseni |
| Orasti |
| Orastie |
| Orastioara de Jos |
| Orastioara de Sus |
| Orasul Victoria |
| Orheiu Bistritei |
| Orlat |
| Orman |
| Ormenis (bei Tirnaveni) |
| Ormenis (bei Turda) |
| Ormenis (bei Sfintu Gheorghe) |
| Ormindea |
| Oroiu de Cimpie |
| Orosfaia |
| Orosia |
| Orsova |
| Osorhel |
| Ostrov |
| Ostrovel |
| Ostrovu Mic |
| Oteni |
| Ozd |
| Ozun |
| Ozunca-Bai |
- |
deutsche Ortsnamen
| - |
| Unterwardein |
| Oberwardein |
| - |
| - |
| - |
| - |
| - |
| - |
| - |
| - |
| Salzdorf |
| - |
| - |
| Miereschhall |
| Salzburg |
| Grubendorf |
| - |
| - |
| - |
| - |
| - |
| Oderhellen |
| - |
| - |
| - |
| - |
| - |
| Steindorf |
| - |
| - |
| - |
| - |
| - |
| - |
|
| - |
| Beschenbach |
| Kleinkerz |
| - |
| - |
| Diesendorf |
| - |
| Broos |
| Unter-Brooserbach |
| Ober-Broserbach |
| Victoria-Stadt |
| Burghalle |
| Winsberg |
| - |
| Irmesch |
| - |
| - |
| - |
| - |
| Reußwald |
| - |
| - |
| - |
| Groß-Rodendorf |
| - |
| Klein-Rodendorf |
| - |
| - |
| Auendorf |
| - |
|
ungarische Ortsnamen
| Marosorbo |
| Alsomarosvaradja |
| Felsömarosvaradja |
| Racatesu |
| - |
| Abrahamfalva |
| Obrazsa |
| - |
| - |
| Homorodokland |
| Aklos |
| Desakna |
| Alsosofalva |
| Felsöofalva |
| Marosujvar |
| Vizakna |
| Kisakna |
| - |
| Nagyoklos |
| Felsöaklos |
| - |
| - |
| Szekelyudvarhely |
| Vamosudvarhely |
| Vadveram |
| Marosugra |
| - |
| - |
| Köaljaohaba |
| - |
| - |
| - |
| Vajasd |
| Ojtoz |
| Ozsdola |
| - |
| Oltszem |
| Bessimbak |
| - |
| Keresztes |
| - |
| Varosfalva |
| Orest |
| Szaszvaros |
| Alsovarosvis |
| Felsövarosviz |
| - |
| Varhely |
| Orlat |
| Ormany |
| Szaszörmenyes |
| Marosörmenyes |
| Ürmös |
| - |
| Mezöuraly |
| Oroszfaja |
| Marosoroszi |
| Görgenyorsova |
| Erdövasarhely |
| Nagyostro |
| - |
| Kisosztro |
| Ocfalva |
| Magyarozd |
| Uzon |
| Uzonkafürdö |
|
