| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| OctÁVia | - | Portuguese | ||
| OctÁVio | - | Portuguese | ||
| Odilon | - | French, Portuguese | ||
| OfÉLia | - | Portuguese | ||
| OlÍVia | - | Portuguese, Slovak, Hungarian | ||
| Olavo | - | Portuguese | ||
| OlegÁRio | - | Portuguese | ||
| Ondina | - | Portuguese, Italian | ||
| Oscar | AHS-kər (English) | English, Irish, Portuguese (Brazilian), Italian, Swedish, Norwegian, Danish, Dutch, French, Irish My | ||
| Osvaldo | - | Spanish, Italian, Portuguese | ||
| OtÁVia | - | Portuguese (Brazilian) | ||
| OtÁVio | - | Portuguese (Brazilian) | ||
| OvÍDio | - | Portuguese |