- Oatquarter
- Oghil
- Oghil
- Oghly Island
- Oileán Dá Bhranóg
- Oileán na Tuí
- Oldstreet
- Omey Island
- Onaght
- Ormond View
- Oughterard
- Oultort
- Ourtnagapple
- Owenbristy
- Ower
- Oxgrove Bridge
- Oghill Cross Roads
- Orcusconny
- Oldcastle
- Ooghwee
- Oughtagh
- Owenboy
- Owenduff
- Owenduff
- Oxford
- Oranmore
- O’Reilly’s Island
- Old Town
- Oran
- Ogham