| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| Onesimos | - | Ancient Greek, Biblical Greek | ||
| Onesiphoros | - | Ancient Greek, Biblical Greek | ||
| Origenes | - | Ancient Greek | ||
| Ortwin | AWRT-veen (German) | German, Ancient Germanic | ||
| Othmar | - | German, Czech, Ancient Germanic | ||
| Otho | - | Ancient Roman | ||
| Otmar | - | German, Ancient Germanic | ||
| Otto | AW-to (German), AH-to (English), OT-to (Finnish) | German, Swedish, Norwegian, Danish, Dutch, English, Finnish, Ancient Germanic | ||
| Ovidius | - | Ancient Roman |