- O'Brien, Charlotte Grace (2, 1845 to 1909)
- O'Brien, Cyril J. (1, 1919 to 2011)
- O'Brien, Fitz-James (2, 1828 to 1862)
- O'Brien, James (1, 1842 to 1930)
- O'Brien, Pat (1, 1890 to 1920)
- O'Connor, William Frederick Travers (1, 1870 to 1943)
- O'Donnell, Elliott (7, 1872 to 1965)
- O'Donnell, T. C. (1, 1881 to 1962)
- O'Hagan, Thomas (1, 1855 to 1939)
- O'Higgins, Harvey Jerrold (1, 1876 to 1929)
- O'Neill, Elizabeth (3, 1877 to 1951)
- O'Neill, Eugene (3, 1888 to 1953)
- O'Shaughnessy, William Brooke (1, 1809 to 1889)
- O'Sullivan, Vincent (3, 1868 to 1940)
- Oakley, Nancy (1, fl.1923)
- Ober, Frederick A. (1, 1849 to 1913)
- Obligado, Pastor Servando (1, 1842 to 1924)
- Ocean Spray Cranberries, Inc. (1, 0 to 0)
- Odle, E. V. (1, 1890 to 1942)
- Oehlenschläger, Adam (2, 1779 to 1850)
- Oesterley, W. O. E. (1, 1866 to 1950)
- of Kronstadt, Saint John (1, 1829 to 1908)
- of Nineveh, Isaac (1, 613 to 700)
- Ogawa, Mimei (1, 1882 to 1961)
- Ogden, George W. (2, 1871 to 1966)
- Ogden, Ruth (2, 1853 to 1927)
- Okakura, Kakuzō (3, 1862 to 1913)
- Oklahoma Commission to Study the Tulsa Race Riot of 192 (1)
- Olcott, Frances Jenkins (3, 1872 to 1963)
- Olcott, H. S. (1, 1832 to 1907)
- Oldmeadow, Ernest (1, 1867 to 1949)
- Olevianus, Caspar (1, 1536 to 1587)
- Oliphant, Laurence (1, 1829 to 1888)
- Oliphant, Margaret O. (16, 1828 to 1897)
- Oliveira Cézar, Filiberto de (4, 1856 to 1910)
- Oliver, Fredrick Spencer (1, 1866 to 1899)
- Oller, Narcís (1, 1846 to 1930)
- Olmsted, Frederick Law (1, 1822 to 1903)
- Olney Kirk, Ellen W. (1, 1842 to 1928)
- Olterdissen, Alphonse (1, 1865 to 1923)
- Oman, Charles William Chadwick (5, 1860 to 1946)
- Ongley, Byron (1, 1876 to 1915)
- Onions, Oliver (5, 1873 to 1961)
- Opie, Amelia (1, 1769 to 1853)
- Oppenheim, E. Phillips (22, 1866 to 1946)
- Optic, Oliver (12, 1822 to 1897)
- Orczy, Baroness Emma (25, 1865 to 1947)
- Origen of Alexandria (7, 184 to 253)
- Orkan, Władysław (1, 1875 to 1930)
- Orléans, Charles d' (1, 1394 to 1465)
- Ormondroyd, Edward (1, 1925 to alive)
- Orr, James (3, 1844 to 1913)
- Orr, Lyndon (1)
- Ortega y Frías, Ramón (1, 1825 to 1883)
- Orton, James (1, 1830 to 1877)
- Osborn, Chase Salmon (1, 1860 to 1949)
- Osborne, Dorothy (1, 1627 to 1695)
- Osborne, Duffield (1, 1858 to 1917)
- Osborne, Selleck (1, 1783 to 1826)
- Osbourne, Lloyd (3, 1868 to 1947)
- Osler, Sir William (3, 1849 to 1919)
- Osman, Eaton G. (1, 1853 to 1929)
- Ossendowski, Ferdinand (1, 1876 to 1945)
- Ossietzky, Carl von (6, 1889 to 1938)
- Ostrander, Isabel (3, 1883 to 1924)
- Ostwald, Wilhelm (1, 1853 to 1932)
- Otis, James (4, 1848 to 1912)
- Otto-Peters, Louise (1, 1819 to 1895)
- Ottolengui, Rodrigues (1, 1861 to 1937)
- Otway, Thomas (1, 1652 to 1685)
- Ouida (4, 1839 to 1908)
- Overton, Grant M. (1, 1887 to 1930)
- Ovidius Naso, Publius (Ovid) (8, c. 43 BC to 18 AD)
- Owen, John (7, 1616 to 1683)
- Owen, Catherine (2, 1889)
- Owen, Everard (1, 1860 to 1949)
- Owen, Robert Dale (1, 1801 to 1877)
- Owen, Wilfred (5, 1893 to 1918)
- Oxenford, John (1, 1812 to 1877)
- Oxley, James Macdonald (1, 1855 to 1907)
- Oyen, Henry (1, 1883 to 1921)
- Ozaki, Milton K. (1, 1913 to 1989)
- Ozaki, Yei Theodora (3, 1871 to 1932)