| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| Oana | WAH-nah | Romanian | ||
| Octavia | ahk-TAYV-ee-ə (English) | English, Spanish, Ancient Roman | ||
| Octavian | ahk-TAYV-ee-ən (English) | History, Romanian | ||
| Octavianus | - | Ancient Roman | ||
| Octavius | ahk-TAYV-ee-əs (English) | Ancient Roman | ||
| Olimpia | aw-LEEM-pyah (Polish) | Italian, Spanish, Romanian, Polish, Hungarian | ||
| Oliva | - | Late Roman | ||
| Otho | - | Ancient Roman | ||
| Ovidiu | - | Romanian | ||
| Ovidius | - | Ancient Roman | ||
| Owain | O-wien (Welsh, Welsh Mythology) | Welsh, Welsh Mythology, Arthurian Romance |