| Onur | Man | Turkish Names |
| Onyekachukwu | Unisex | African - Igbo Names |
| Onyx | Unisex | English Names |
| Ooko | Man | African - Luo of Kenya Names |
| Oona | Woman | Irish Names |
| Oonagh | Woman | Irish Names |
| Oorjita | Woman | Indian Names |
| Oormi | Woman | Indian Names |
| Ooyu | Unisex | Native American - Coos Names |
| Opa | Woman | Native American - Choctaw Names |
| Opal | Woman | Indian Names |
| Opaque | Unisex | French Names |
| Opelenge | Woman | African - Yoruba Names |
| Ophelia | Woman | Greek Names |
| Ophélie | Woman | French Names |
| Ophira | Woman | Hebrew Names |
| Ophrah | Man | Hebrew Names |
| Oprah | Woman | Hebrew Names |
| Or | Unisex | Hebrew Names |
| Ora | Woman | Spanish Names |