| Name | What | Region | Country | Lat | Long | Elev ft. | Pop est |
|---|---|---|---|---|---|---|---|
| Oilean Eadai | city | Mayo | Ireland | 53.83 | -9.40 | 239 | 1519 |
| Oldcastle | city | Mayo | Ireland | 53.95 | -9.03 | 91 | 1732 |
| Ooghwee | city | Mayo | Ireland | 54.30 | -10.01 | 344 | 939 |
| Oughtagh | city | Mayo | Ireland | 53.92 | -9.10 | 147 | 1906 |
| Owenboy | city | Mayo | Ireland | 54.08 | -9.43 | 206 | 665 |
| Owenduff | city | Mayo | Ireland | 54.03 | -9.68 | 370 | 146 |
| Owenduff | city | Mayo | Ireland | 53.95 | -9.84 | 16 | 437 |