| Oceanus | | - | Greek Mythology (Latinized) | |
| Oden | | - | Norse Mythology | |
| Odin | | O-din | Norse Mythology, English (Modern) | |
| Odysseus | | o-DIS-ee-əs (English) | Greek Mythology | |
| Oedipus | | ED-i-pəs (English), EED-i-pəs (English) | Greek Mythology (Latinized) | |
| Oenone | | - | Greek Mythology (Latinized) | |
| Oidipous | | - | Greek Mythology | |
| Oinone | | - | Greek Mythology | |
| OisÍN | | OSH-een, USH-een | Irish, Irish Mythology | |
| Okeanos | | - | Greek Mythology | |
| Orestes | | o-RES-teez (English) | Greek Mythology | |
| Orion | | o-RIE-ən (English) | Greek Mythology | |
| Ormazd | | OR-mazd | Persian Mythology | |
| Orpheus | | AWR-fee-əs (English) | Greek Mythology | |
| Orvar | | - | Swedish, Norse Mythology | |