| Ortwin | | AWRT-veen (German) | German, Ancient Germanic | |
| Osamu | | o-sah-moo | Japanese | |
| Osanna | | - | Italian (Rare) | |
| Osbeorn | | - | Anglo-Saxon | |
| Osberht | | - | Anglo-Saxon | |
| Oscar | | AHS-kər (English) | English, Irish, Portuguese (Brazilian), Italian, Swedish, Norwegian, Danish, Dutch, French, Irish My | |
| Osgar | | - | Anglo-Saxon | |
| Osip | | O-seep | Russian | |
| Osiris | | o-SIE-ris (English) | Egyptian Mythology (Hellenized) | |
| Oskar | | AWS-kahr (German, Polish) | Swedish, Norwegian, Danish, German, Polish | |
| Oskars | | - | Latvian | |
| Osmund | | - | Anglo-Saxon | |
| Osvald | | - | Swedish, Norwegian, Danish | |
| Osvaldo | | - | Spanish, Italian, Portuguese | |
| Oswald | | AHZ-wawld (English), AWS-vahlt (German) | English, German, Anglo-Saxon | |