| Osborne | | AHZ-born | English | |
| Osbourne | | AHZ-born | English (Rare) | |
| Oscar | | AHS-kər (English) | English, Irish, Portuguese (Brazilian), Italian, Swedish, Norwegian, Danish, Dutch, French, Irish My | |
| Osiris | | o-SIE-ris (English) | Egyptian Mythology (Hellenized) | |
| Osmond | | AHZ-mənd | English (Rare) | |
| Oswald | | AHZ-wawld (English), AWS-vahlt (German) | English, German, Anglo-Saxon | |
| Oswin | | AHZ-win | English (Rare) | |
| Othmar | | - | German, Czech, Ancient Germanic | |
| Otho | | - | Ancient Roman | |
| Otis | | O-tis | English | |
| Otmar | | - | German, Ancient Germanic | |
| Otto | | AW-to (German), AH-to (English), OT-to (Finnish) | German, Swedish, Norwegian, Danish, Dutch, English, Finnish, Ancient Germanic | |
| Ottoline | | - | English (Rare) | |
| Ovidius | | - | Ancient Roman | |
| Owen (1) | | O-ən (English) | Welsh, English | |