| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| 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 | ||
| Osiris | o-SIE-ris (English) | Egyptian Mythology (Hellenized) | ||
| Ourania | - | Greek Mythology | ||
| Ouranos | - | Greek Mythology | ||
| Owain | O-wien (Welsh, Welsh Mythology) | Welsh, Welsh Mythology, Arthurian Romance | ||
| Pallas (1) | PAL-əs (English) | Greek Mythology |