- O'Beirne, Francis (Mr) 1833 - 1899
- O'Beirne, James (Mr) 1816 - 1895
- O'Brien, Bill (Mr) January 25, 1929 -
- O'Brien, Cornelius (Mr) 1782 - 1857
- O'Brien, Edward (Sir) April 17, 1773 - March 13, 1837
- O'Brien, Ignatius (Mr) July 31, 1857 - September 10, 1930
- O'Brien, James (Mr) 1806 - 1881
- O'Brien, James (Mr) 1828 - May 23, 1905
- O'Brien, John (Mr) 1794 - 1855
- O'Brien, Kendal (Mr) 1849 - November 21, 1909
- O'Brien, Leslie (Mr) February 8, 1908 - November 24, 1995
- O'Brien, Lucius (Sir) 1800 - 1872
- O'Brien, Mike (Mr) June 19, 1954 -
- O'Brien, Murrough (Mr) - February 10, 1808
- O'Brien, Ossie (Mr) April 6, 1928 - March 6, 1997
- O'Brien, Patrick (Mr) 1823 - 1895
- O'Brien, Patrick (Mr) 1853 - July 12, 1917
- O'Brien, Patrick (Mr) 1835 - January 10, 1911
- O'Brien, Peter (Mr) June 29, 1842 - September 7, 1914
- O'Brien, Stephen (Mr) April 1, 1957 -
- O'Brien, Timothy (Mr) - 1862
- O'Brien, Tom (Mr) August 17, 1900 - May 5, 1970
- O'Brien, William (Mr) 1852 - February 25, 1928
- O'Brien, William (Mr) 1803 - 1864
- O'Bryen, William (Mr) - August 21, 1846
- O'Byrne, William (Mr) 1823 - 1896
- O'Callaghan, Cornelius (Hon.) 1809 - 1849
- O'Callaghan, Cornelius (Mr) October 2, 1775 - May 30, 1857
- O'Callaghan, George (Mr) March 16, 1815 - October 29, 1898
- O'Callaghan, James (Mr) October 17, 1743 - February 14, 1836
- O'Callaghan, William (Hon.) 1852 - 1877
- O'Cathain, Detta (Ms) February 3, 1938 -
- O'Clery, Keyes (Mr) 1849 - May 22, 1913
- O'Connell, Charles (Mr) - 1877
- O'Connell, Daniel (Mr) 1775 - 1847
- O'Connell, Daniel (Mr) 1815 - 1897
- O'Connell, John (Mr) 1810 - 1858
- O'Connell, Maurice (Mr) 1803 - 1853
- O'Connell, Maurice (Mr) 1801 - June 18, 1853
- O'Connell, Morgan (Mr) 1811 - 1875
- O'Connell, Morgan (Mr) 1804 - 1885
- O'Connor, Arthur (Mr) 1888 - May 2, 1950
- O'Connor, Arthur (Mr) 1844 - March 30, 1923
- O'Connor, Feargus (Mr) 1796 - 1855
- O'Connor, James (Mr) 1836 - March 12, 1910
- O'Connor, John (Mr) 1850 - October 27, 1928
- O'Connor, John (Mr) 1837 - 1891
- O'Connor, T.P. (Mr) 1848 - November 18, 1929
- O'Connor, Terence (Mr) 1891 - May 8, 1940
- O'Conor, Denis (Mr) 1840 - 1883
- O'Conor Don, (The) 1838 - 1906
- O'Conor Don, (The) 1794 - 1847
- O'Conor, Owen (Mr) 1763 - 1831
- O'Doherty, James (Mr) 1848 - 1932
- O'Doherty, Joseph (Mr) 1891 -
- O'Doherty, Kevin (Mr) 1823 - July 15, 1905
- O'Doherty, Philip (Mr) 1871 - February 6, 1926
- O'Doherty, William (Mr) 1868 - May 18, 1905
- O'Donnell, Charles (Mr) 1850 - December 3, 1934
- O'Donnell, Frank (Mr) 1848 - November 2, 1916
- O'Donnell, John (Mr) 1870 - 1920
- O'Donnell, Thomas (Mr) 1872 - June 11, 1943
- O'Donoghue of the Glens, (The) 1833 - 1889
- O'Donovan, William (Dr) 1886 - January 13, 1955
- O'Dowd, John (Mr) 1856 - 1937
- O'Driscoll, Florence (Mr) 1858 - January 6, 1939
- O'Dwyer, Andrew (Mr) - 1877
- O'Flaherty, Anthony (Mr) 1800 - 1866
- O'Gorman Mahon, (The) 1803 - 1891
- O'Gorman, Purcell (Mr) 1818 - 1888
- O'Grady, James (Mr) 1866 - December 10, 1934
- O'Grady, Standish (Lieut-Colonel Hon.) 1795 - 1848
- O'Grady, Standish (Mr) December 26, 1792 - July 22, 1848
- O'Hagan, Thomas (Mr) 1812 - 1885
- O'Halloran, Michael (Mr) August 20, 1929 - 1999
- O'Hanlon, Thomas (Mr) 1837 - 1897
- O'Hara, Eddie (Mr) October 1, 1937 -
- O'Hara, James (Mr) 1796 - 1838
- O'Hare, Patrick (Mr) 1849 - November 1, 1917
- O'Hea, Patrick (Mr) 1848 - 1925
- O'Higgins, Brian (Mr) 1882 - March 10, 1963
- O'Higgins, Kevin (Mr) 1892 - July 10, 1927
- O'Keefe, Francis (Mr) 1856 - April 21, 1909
- O'Keefe, John (Mr) 1827 - 1877
- O'Keefe, Patrick (Mr) -
- O'Kelly, Conor (Mr) 1873 -
- O'Kelly, Edward (Mr) 1846 - July 22, 1914
- O'Kelly, James (Mr) 1845 - December 22, 1916
- O'Kelly, John (Mr) 1872 - March 26, 1957
- O'Kelly, Sean (Mr) 1882 - November 23, 1966
- O'Leary, Daniel (Mr) 1875 - December 23, 1954
- O'Leary, William (Dr) 1839 - 1880
- O'Loghlen, Bryan (Sir) 1828 - October 31, 1905
- O'Loghlen, Colman (Sir) 1819 - 1877
- O'Loghlen, Michael (Mr) 1789 - 1842
- O'Mahony, John (Mr) 1864 - November 28, 1934
- O'Maille, Padraic (Mr) 1878 - January 19, 1946
- O'Malley, Brian (Mr) January 22, 1930 - April 6, 1976
- O'Malley, William (Mr) 1857 - 1939
- O'Mara, James (Mr) 1873 - November 21, 1948
- O'Mara, Stephen (Mr) 1844 - July 26, 1926
- O'Neill, Arthur (Hon.) 1876 - November 4, 1914
- O'Neill, Charles (Dr) 1849 - January 14, 1918
- O'Neill, Edward (Hon.) 1839 - November 19, 1928
- O'Neill, Hugh (Hon.) 1883 - 1982
- O'Neill, Hugh (Mr) June 14, 1939 -
- O'Neill, John (Hon.) 1780 - 1855
- O'Neill, John (Mr) 1888 - April 20, 1953
- O'Neill, John (Mr) 1799 -
- O'Neill, Martin (Mr) January 6, 1945 -
- O'Neill, Michael (Mr) October 7, 1910 - October 4, 1976
- O'Neill, Onora (Ms) August 23, 1941 -
- O'Neill, Phelim (Mr) November 2, 1909 - 1994
- O'Neill, Raymond (Mr) September 1, 1933 -
- O'Neill, Robert (Hon.) 1845 - July 25, 1910
- O'Neill, Terence (Mr) September 10, 1914 - June 12, 1990
- O'Neill, William (Mr) - April 18, 1883
- O'Reilly, Myles (Major) 1825 - 1880
- O'Reilly, William (Mr) - 1844
- O'Shaughnessy, Patrick (Mr) 1872 - December 29, 1920
- O'Shaughnessy, Richard (Mr) 1842 - 1918
- O'Shea, William (Mr) 1840 - 1905
- O'Sullivan, Eugene (Mr) 1879 - May 19, 1942
- O'Sullivan, Timothy (Mr) 1879 - August 15, 1950
- O'Sullivan, William (Mr) 1829 - 1887
- Oakes, Gordon (Mr) June 22, 1931 - August 14, 2005
- Oakes, James (Mr) 1821 - 1901
- Oakeshott, Matthew (Mr) January 10, 1947 -
- Oakley, Thomas (Mr) 1879 - April 4, 1936
- Oakshott, Hendrie (Mr) November 8, 1904 - February 1, 1975
- Oaten, Mark (Mr) March 8, 1964 -
- Oddy, John (Mr) 1867 - February 20, 1921
- Odey, George (Mr) April 21, 1900 - 1985
- Ogden, Eric (Mr) August 23, 1923 - 1997
- Ogden, Frederick (Mr) 1871 - April 24, 1933
- Ogilvie-Grant, Francis (Hon.) 1778 - 1853
- Ogilvie-Grant, Francis (Mr) 1814 - 1840
- Ogilvie-Grant, Ian (Mr) March 20, 1939 -
- Ogilvy, David (Mr) July 18, 1893 - December 28, 1968
- Ogilvy, David (Mr) May 17, 1926 -
- Ogilvy, Donald (Mr) May 1, 1788 - December 30, 1863
- Ogilvy, John (Sir) 1803 - 1890
- Ogilvy, William (Mr) 1794 - April 10, 1871
- Oglander, William (Sir) September 13, 1769 - January 17, 1852
- Ogle, Charles (Sir) May 24, 1775 - June 16, 1858
- Ogle, Henry (Mr) 1762 - February 18, 1823
- Ogle, Saville (Mr) 1811 - 1854
- Oldfield, John (Mr) 1900 -
- Oldfield, William (Mr) 1881 - November 16, 1961
- Oldroyd, Mark (Mr) 1843 - July 5, 1927
- Oliphant, Laurence (Mr) 1791 - 1862
- Oliphant, Laurence (Mr) 1829 - 1888
- Oliveira, Benjamin (Mr) 1806 - 1865
- Oliver, Charles (Mr) 1763 - October 10, 1817
- Oliver, George (Mr) 1888 - 1984
- Oliver, John (Mr) April 14, 1935 -
- Oliver, Laurence (Mr) May 22, 1907 - July 11, 1989
- Oliver, Peter (Mr) March 7, 1921 - October 17, 2007
- Oliver, Philip (Mr) 1884 - April 12, 1954
- Olivier, Sydney (Mr) April 16, 1859 - February 15, 1943
- Olner, Bill (Mr) May 9, 1942 -
- Oman, Charles (Professor Sir) 1860 - June 23, 1946
- Ommanney, Francis (Mr) 1774 - November 7, 1840
- Onions, Alfred (Mr) 1858 - July 6, 1921
- Onslow, Arthur (Mr) August 3, 1759 - October 8, 1833
- Onslow, Cranley (Mr) June 8, 1926 - March 13, 2001
- Onslow, Denzil (Mr) 1839 - 1908
- Onslow, George (Mr) - May 17, 1814
- Onslow, Guildford (Mr) 1814 - 1882
- Onslow, Michael (Mr) February 28, 1938 -
- Onslow, Thomas (Mr) October 7, 1778 - July 7, 1861
- Onslow, Thomas (Mr) March 15, 1754 - February 22, 1827
- Öpik, Lembit (Mr) March 2, 1965 -
- Oppenheim, Philip (Mr) March 20, 1956 -
- Oppenheim, Sally (Mrs) July 26, 1930 -
- Oram, Albert (Mr) August 13, 1913 - September 4, 1999
- Orbach, Maurice (Mr) July 13, 1902 - April 24, 1979
- Orchard, Paul (Mr) June 26, 1739 - March 1, 1812
- Ord, William (Mr) 1781 - 1855
- Ord, William (Mr) 1803 - 1838
- Orde, John (Sir) December 22, 1751 - February 19, 1824
- Orde-Powlett, Nigel (Mr) March 26, 1900 - June 15, 1963
- Orde-Powlett, Richard (Mr) July 11, 1929 - July 29, 2001
- Orde-Powlett, Thomas (Mr) August 30, 1740 - July 30, 1807
- Orde-Powlett, William (Hon.) 1869 - December 11, 1944
- Orde-Powlett, William (Mr) February 24, 1818 - November 7, 1895
- Orde-Powlett, William (Mr) January 31, 1845 - August 14, 1922
- Orde-Powlett, William (Mr) October 31, 1782 - July 13, 1850
- Organ, Diana (Mrs) February 21, 1952 -
- Orme, Stan (Mr) April 5, 1923 - April 27, 2005
- Ormelie, (Earl of) 1796 - 1862
- Ormiston, Thomas (Mr) 1878 - January 15, 1937
- Ormsby, Charles (Mr) April 23, 1767 - March 3, 1818
- Ormsby-Gore, Francis (Mr) March 13, 1954 -
- Ormsby-Gore, George (Hon.) 1855 - May 8, 1938
- Ormsby-Gore, John (Mr) 1816 - 1876
- Ormsby-Gore, Seymour (Hon.) 1863 - November 10, 1950
- Ormsby-Gore, William (Mr) 1819 - June 27, 1904
- Ormsby-Gore, William (Mr) 1885 - February 14, 1964
- Ormsby-Gore, William (Mr) 1779 - 1860
- Ormsby-Gore, William (Mr) May 20, 1918 - 1985
- Orr, Lawrence (Mr) September 16, 1918 - 1990
- Orr-Ewing, Archibald (Mr) 1818 - 1893
- Orr-Ewing, Charles (Mr) 1860 - December 24, 1903
- Orr-Ewing, Ian (Mr) 1893 - April 27, 1958
- Orr-Ewing, Ian (Mr) February 10, 1912 - August 19, 1999
- Osbaldston, George (Mr) December 26, 1786 - August 1, 1866
- Osborn, John (Mr) December 3, 1772 - August 28, 1848
- Osborn, John (Mr) December 14, 1922 -
- Osborne, Cyril (Mr) 1898 - August 31, 1969
- Osborne, Francis (Lord) October 18, 1777 - February 15, 1850
- Osborne, Francis (Mr) May 21, 1798 - May 4, 1859
- Osborne, Francis (Mr) September 16, 1884 - March 20, 1964
- Osborne, George (Mr) May 23, 1971 -
- Osborne, George (Mr) August 11, 1828 - December 23, 1895
- Osborne, George (Mr) July 16, 1802 - August 8, 1872
- Osborne, George (Mr) July 21, 1775 - July 10, 1838
- Osborne, John (Mr) March 12, 1901 - July 26, 1963
- Osborne, Sandra (Ms) February 23, 1956 -
- Ossulston, (Lord) 1810 - 1899
- Oswald, Alexander (Mr) 1811 - 1868
- Oswald, James (Mr) 1777 - 1853
- Oswald, James (Mr) 1838 - September 14, 1908
- Oswald, Richard (Mr) 1771 - 1841
- Oswald, Thomas (Mr) May 1, 1904 - 1990
- Ottaway, Richard (Mr) May 24, 1945 -
- Otter, Francis (Mr) 1831 - 1895
- Otway, Arthur (Sir) 1822 - June 8, 1912
- Ouseley, Herman (Mr) March 24, 1945 -
- Outhwaite, Robert (Mr) 1869 - November 6, 1930
- Ovenden, John (Mr) 1942 -
- Overend, William (Mr) 1809 - 1884
- Owen, Albert (Mr) August 10, 1959 -
- Owen, David (Dr) July 2, 1938 -
- Owen, Edward (Sir) 1771 - October 8, 1849
- Owen, Goronwy (Major) 1881 - September 26, 1963
- Owen, Hugh (Mr) 1729 - January 23, 1809
- Owen, Hugh (Sir) September 12, 1782 - August 8, 1809
- Owen, Hugh (Sir) 1803 - 1891
- Owen, Humphrey (Mr) September 27, 1905 - 1979
- Owen, Idris (Mr) 1912 - 2004
- Owen, John (Sir) 1776 - 1861
- Owen, Thomas (Mr) 1840 - 1898
- Owen, William (Mr) February 18, 1901 - 1981
- Oxburgh, Ernest (Mr) November 2, 1934 -
- Oxmantown, (Lord) 1800 - 1867
- of Mar, Margaret (Ms) September 19, 1940 -
14,855 people in total