Agents persons, peoples and institutions
- s. xx–xxi
O'Donnell (Red Hugh) See: Aodh Ruadh Ó Domhnaill
- s. xx–xxi
- 1806–1861
- d. 1934
O'Dwyer (Denis) See: Donnchadh Ó Duibhir
- s. xix2–xx1
- 1627/30–1716/18
- 1876–1942
- c.1760–1814
O'Flynn (Denis) See: Donnchadh Ó Floinn [d. 1830]
- s. xx–xxi
- s. xix2–xx1
O'Gorman (Maurice) See: Muiris Ó Gormáin
- d. 1915
- d. 18 October 1899
- s. xx–xxi
- d. 1807
O'Hanlon (Valentine) See: Vailintín Ó hAnluain
O'Hussey (Bonaventura) See: Giolla Brighde Ó hEodhusa