| 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 | |
| Paschalis | | - | Late Roman | |
| Pastor | | - | Spanish, Late Roman | |
| Patricia | | pə-TRISH-ə (English), pah-TREE-thyah (Spanish), pah-TREE-syah (Latin American Spanish), pah-TREE-tsee-ah (German) | English, Spanish, German, Late Roman | |
| Patricius | | - | Late Roman | |