O
- Offington Stream, County Dublin
- Ollatrim River, North Tipperary
- Owenaher River, County Louth
- Owenbehy River, County Mayo
- Owenbrean*
- Owendoher River, County Dublin
- Owenreagh*
- Owenkillew*
- Owenroe
- Oona
Read more about this topic: Rivers Of Northern Ireland