| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| Onesiphoros | - | Ancient Greek, Biblical Greek | ||
| Onesiphorus | - | Biblical, Biblical Latin | ||
| Onfroi | - | Medieval French | ||
| Oni | - | Western African, Yoruba | ||
| Onisim | - | Russian (Rare) | ||
| Onisimu | - | Old Church Slavic | ||
| Onyeka | - | Western African, Igbo | ||
| Onyekachi | - | Western African, Igbo | ||
| Onyekachukwu | - | Western African, Igbo | ||
| Opaline | O-pə-leen | English (Rare) | ||
| Opeyemi | - | Western African, Yoruba | ||
| Ophelia | o-FEEL-yə (English) | English, Literature | ||
| Ophir | - | Biblical | ||
| Ophiuchus | - | Astronomy | ||
| Ophrah | AWF-rə (English) | Biblical |