| Name | Gender | Pronounced | Usage | Popularity |
|---|---|---|---|---|
| Odharnait | - | Irish | ||
| OdhrÁN | O-rawn | Irish | ||
| Odran | - | Irish | ||
| OisÍN | OSH-een, USH-een | Irish, Irish Mythology | ||
| OnÓRa | - | Irish | ||
| Oona | OO-na (Irish), O:-nah (Finnish) | Irish, Finnish | ||
| Oonagh | OO-na | Irish | ||
| Oran | O-ran | Irish | ||
| Orla (1) | OR-la | Irish | ||
| Orlagh | - | Irish | ||
| Orna (1) | - | Irish | ||
| Ornat | - | Irish | ||
| Orrin | - | Irish | ||
| Oscar | AHS-kər (English) | English, Irish, Portuguese (Brazilian), Italian, Swedish, Norwegian, Danish, Dutch, French, Irish My | ||
| Osheen | - | Irish |