{|

| style="width=30em" |   | align=right | page |- |O'Brien, William Smith (1803-1864) |332 |- |O'Brolchain, Flaibhertach (d. 1175) |337 |- |O'Bruadair, David (fl. 1650-1694) |338 |- |O'Bryan, William (1778-1868) |339 |- |O'Bryen, Dennis (1755-1832) |340 |- |O'Bryen, Edward (1754?-1808) |340 |- |O'Byrne, Fiagh Mac Hugh (1544?-1597) |341 |- |O'Cahan or O'Kane, Sir Donnell Ballagh, or 'the freckled' (d. 1617?) |344 |- |O'Callaghan, Edmund Bailey (1797-1880) |345 |- |O'Callaghan, John Cornelius (1805-1883) |346 |- |O'Callaghan, Sir Robert William (1777-1840) |346 |- |O'Caran, Gilla-an-Choimhdedh (d. 1180) |347 |- |O'Carolan or Carolan, Torlogh (1670-1738) |347 |- |O'Carroll, Maolsuthain (d. 1031) |349 |- |O'Carroll, Margaret (d. 1451) |350 |- |Occam, Nicholas of (fl. 1280) |350 |- |Occam, William (d. 1349? ). See Ockham. | |- |Occleve, Thomas (1370?-1450?). See Hoccleve. | |- |O'Cearbhall (d. 888), lord of Ossory. See Cearbhall. | |- |O'Cearnaidh, Brian (1567-1610). See Kearney, Barnabas. | |- |Ochiltree, second Baron. See Stewart, Andrew (d. 1568). | |- |Ochiltree, Michael (fl. 1425-1445) |350 |- |Ochino, Bernardino (1487-1564) |350 |- |Ochs, Johann Rudolph (1673-1749). See under Ochs or Ocks, John Ralph. | |- |Ochs or Ocks, John Ralph (1704-1788) |353 |- |Ochterlony, Sir David (1758-1825) |353 |- |Ockham, Barons of. See King, Peter, first Lord King (1669-1734); King, Peter, seventh Lord King (1776-1833). | |- |Ockham, Nicholas of (fl. 1280). See Occam. | |- |Ockham or Occam, William (d. 1349 ?) |357 |- |Ockland, Christopher (d. 1590?). See Ocland. | |- |Ockley, Simon (1678-1720) |362 |- |Ocks, John Ralph (1704-1788). See Ochs. | |- |Ocland, Christopher (d. 1590?) |365 |- |O'Clery, Cucoigcriche (d. 1664). See under O'Clery, Lughaidh. | |- |O'Clery, Lughaidh (fl. 1609) |366 |- |O'Clery, Michael (1575-1643) |367 |- |O'Cobhthaigh, Dermot (fl. 1584) |369 |- |O'Connell, Daniel or Daniel Charles, Count (1745?-1833) |370 |- |O'Connell, Daniel (1775-1847) |371 |- |O'Connell, John (1810-1858) |389 |- |O'Connell, Sir Maurice Charles (1812-1879) |390 |- |O'Connell, Sir Maurice Charles Philip (d. 1848) |391 |- |O'Connell, Morgan (1804-1885) |392 |- |O'Connell, Moritz, Baron O'Connell (1740?-1830) |393 |- |O'Connell, Peter (1746-1826) |393 |- |O'Connor. See also O'Conor. | |- |O'Connor, Aedh (d. 1067) |393 |- |O'Connor, Arthur (1763-1852) |394 |- |O'Connor, Bernard, or Brian O'Conor Faly (1490?-1560?) |395 |- |O'Connor, Bernard (1666?-1698). See Connor. | |- |O'Connor, Calvach (1584-1655) |398 |- |O'Connor, Cathal (d. 1010) |398 |- |O'Connor, Cathal (1150?-1224) |398 |- |Connor or O'Conor Faly, Cathal or Charles, otherwise known as Don Carlos (1540-1596). See under O'Connor, Bernard (1490?-1560?) |- |O'Connor, Feargus(1794-1855) |400 |- |O'Connor, Hugh (1617-1669). See under O'Connor, Calvach. | |- |O'Connor, James Arthur (1791-1841) |402 |- |O'Connor, John (1824-1887) |403 |- |O'Connor, John (1830-1889) |403 |- |O'Connor, Luke Smythe (1806-1873) |404 |- |O'Connor, Roderic, or in Irish Ruaidhri (d. 1118) |405 |- |O'Connor, Roderic (1116-1198) |405 |- |O'Connor, Roger (1762-1834) |407 |- |O'Connor, Turlough (1088-1156) |408 |- |O'Conor. See also O'Connor. | |- |O'Conor, Charles (1710-1791) |410 |- |O'Conor, Charles (1764-1828) |412 |- |O'Conor, Matthew (1773-1844) |413 |- |O'Conor, William Anderson (1820-1887) |414 |- |Octa, Ocga, Oht, or Oiric (d. 532?) |414 |- |O'Cullane, John (1754-1816) |415 |- |O'Curry, Eugene (1796-1862). |415 |- |O'Daly, Aengus (d. 1350) |416 |- |O'Daly, Aengus (d. 1617) |417 |- |O'Daly, Daniel or Dominic (1595-1662). See Daly. | |- |O'Daly, Donnchadh (d. 1244) |417 |- |O'Daly, Muiredhach (fl. 1213) |418 |- |Odda. See Odo. | |- |Odell, Thomas (1691-1749) |418 |- |O'Dempsey, Dermot (d. 1193) |419 |- |O'Devany or O'Duane, Cornelius (1533-1612) |419 |- |Odger, George (1820-1877) |420 |- |Odingsells, Gabriel (1690-1734) |421 |- |Odington, Walter, or Walter of Evesham (fl. 1240). See Walter. | |- |Odo or Oda (d. 959) |421 |- |Odo or Odda (d. 1056) |423 |- |Odo (d. 1097) |424 |- |Odo of Canterbury (d. 1200) |426 |- |Odo of Cheriton, or, less familiarly, Sherston (d. 1247) |428 |- |O'Dogherty, Sir Cahir (1587-1608) |429 |- |O'Doherty, William James (1835-1868) |431 |- |O'Doirnin, Peter (1682-1768) |431 |- |O'Domhnuill, William (d. 1628). See Daniel. | |- |Odone, William of (d. 1298). See Hothum. | |- |O'Donnel, James Louis (1738-1811) |432 |- |O'Donnell, Calvagh (d. 1566) |432 |- |O'Donnell, Daniel (1666-1735) |434 |- |O'Donnell, Godfrey (d. 1258) |435 |- |O'Donnell, Hugh Balldearg (d. 1704) |435 |- |O'Donnell, Hugh Roe (1571?-1602) |436 |- |O'Donnell, John Francis (1837-1874) |440 |- |O'Donnell, Manus (d. 1564) |441 |- |O'Donnell, Mary Stuart (fl. 1632). See under O'Donnell, Rory, first Earl of Tyrconnel. | |- |O'Donnell, Sir Niall Garv (1569-1626) |443 |- |O'Donnell, Rory, first Earl of Tyrconnel (1575-1608) |444 |- |O'Donovan, Edmund (1844-1883) |447 |- |O'Donovan, John (1809-1861) |448 |- |O'Duane, Cornelius (1533-1612). See O'Devany | |- |O'Dugan, John, the Great (d. 1872) |450 |- |}