- O'Brien, Dermod, P.R.H.A., 1865 - 1945
- O'Brien, Edward, 4th Bt., 1773 - 1837
- O'Brien, Nelly, 1768
- O'Connor, James, 1792? - 1841
- O'Connor, John, 1913 - 2004
- O'Neil, Henry Nelson, A.R.A., 1817 - 1880
- O'Neill, George B., 1828 - 1917
- O'Neill, Moira, 1865 - 1955
- O'Rorke, Brian, R.A., 1901 - 1974
- Oakeley, Herbert, 1830 - 1903
- Oakes, John Wright, A.R.A., 1820 - 1887
- Oakey, D. F., fl. ca. 1847 - 1862?
- Obermayer, Ed., fl. ca. 1863 - 1867?
- Obizzi, Tommaso degli
- Ocean, Humphrey, R.A., 1951 -
- Octavius,, 1779 - 1783
- Odam, Hieronymus
- Oddi, Lavinia Vermiglioli, contessa, fl. ca. 1837?
- Oddoni, Guglielmo, fl. ca. 1615 - 1663?
- Odescalchi, Baldassare d'Erba, duca di Ceri, 1748 - 1810
- Odieuvre, Michel, 1687 - 1756
- Odysseus (Greek mythology)
- Oeser, Adam Friedrich, 1717 - 1799
- Officina Frobeniana
- Officina Janssonio-Waesbergiana (Amsterdam)
- Officina Plantiniana (Antwerp)
- Ogborne, John, 1755 - 1837
- Oggiono, Marco D', ca. 1467 - ca. 1524
- Ôgier, Matt.
- Ogilby, John, 1600 - 1676
- Ogilvy, David, 1742? - 1812
- Oglander, John
- Ogle, George, 1704 - 1746
- Ogle, John William, 1824 - 1905
- Okeover, Leak, 1701 - 1765
- Okeover, Mary, 1696 - 1764
- Okey, Thomas, 1852 - 1935
- Olis, Jan, ca. 1610 - 1676
- Oliver and Boyd (Edinburgh)
- Oliver, Archer James, A.R.A., 1774 - 1842
- Oliver, William Henry, fl.18591890
- Olivier, François, 1487 - 1560
- Olivier, Herbert Armould, 1861 - 1952
- Olivier, Mrs D., fl. 1961
- Ollivant, Alfred, 1798 - 1882
- Ollivier, Emile Edmond, 1800 - 1864
- Ollivier, John, fl. 1837 - 1853
- Olsson, Julius, R.A., 1864 - 1942
- Onfroy, Eugène, before 1765 - 1809
- Ongania, Ferdinando, 1842 - 1911
- Onghena, Karel, 1806 - 1886
- Onions, Oliver, 1873 - 1961
- Onslow, Arthur, 1691 - 1768
- Opie, Amelia, 1769 - 1853
- Opie, John, R.A., 1761 - 1807
- Oporinus, Joannes, 1507 - 1568
- Oppenheim, Charles, fl. 1910
- Oppenheimer, Charles, 1875 - 1961
- Oppenord, Giles Marie, 1672 - 1742
- Oppizzoni, Carlo, Cardinal, 1769 - 1855
- Opstal, Gerard van, 1594 - 1668
- Orazi, Bartolommeo, fl. 1792
- Orazi, Carlo, 1765
- Orazi, Giuseppe, fl. ca. 1752?
- Orazi, Niccolò, fl. 1750 - 1760
- Orcagna,, ca. 1315-20 - 1368
- Orchardson,
- Orchardson, William Quiller, R.A., 1832 - 1910
- Order of St. Michael and St. George
- Order of the Garter
- Ordóñez, Bartolomé, ca. 1485 - ca. 1520
- Orena, Cesare, fl. ca. 1796?
- Orestes (Greek mythology),
- Orford, George Walpole, 3rd Earl of, 1730 - 1791
- Orford, Horace Walpole, 4th Earl of, 1717 - 1797
- Orford, Robert Walpole, 1st Earl of, 1676 - 1745
- Orlandi, Cesare
- Orlandi, Giovanni, fl. 1590-1640
- Orlandi, Orazio
- Orlandi, Pellegrino Antonio, 1660 - 1727
- Orlandini, Stefano, fl. ca. 1758?
- Orléans, Louis-Philippe-Joseph, duc d', 1747 - 1793
- Orleans, Philippe, duc d', Regent of France, 1674 - 1723
- Orme, Daniel, 1766 - 1837
- Orme, Edward, 1775 - 1848
- Ormerod, George, 1785 - 1873
- Ormond, James Butler, 2nd Duke of, 1665 - 1745
- Ornithological Society of London
- Orpen, William, R.A., 1878 - 1931
- Orr, Alexandra, 1827 - 1903
- Orr, Chris, R.A., 1943 -
- Orschall, Johann Christian
- Orsini d'Aragona, Domenico, Prince, Cardinal
- Orsini, Baldassare, 1732 - 1810
- Orsolini, Carlo, ca. 1703 - ca. 1780
- Orti, Girolamo, proveditore of Verona, fl. ca. 1753?
- Ortiz de Villajos, Agustín, 1829 - 1902
- Osborn, J., fl. 1704 - 1743?
- Osborn, John, 1739
- Osborne, Cyril, 1898 - 1969
- Osborne, James, fl. 1792?
- Osborne, Malcolm, R.A., 1880 - 1963
- Osborne, Thomas, ca. 1704? - 1767
- Osborne, Thomas, 1744
- Ossenbeck, Jan van, 1624 - 1674
- Ostade, Adriaen van, 1610 - 1685
- Ostell, Thomas, fl. 1804 - 1808
- Ostell, William, fl. 1836 - 1866
- Osterreich, Matthias, 1716 - 1778
- Osterreichische Nationalbibliothek (Vienna)
- Ostervald,, aîné, fl. ca. 1805 - 1830?
- Oswald, John, fl. 1712 - 1764
- Otley, Edward John, fl. ca. 1840 - 1846?
- Otridge, John, after 1825
- Otridge, William, 1812
- Ottaviani, Giovanni, ca. 1735 - 1808
- Ottley, Henry, 1811 - 1878
- Ottley, Sarah Elizabeth, 1825
- Ottley, William Young, 1771 - 1836
- Otto Henry, Elector Palatine of the Rhine and Count Palatine of Neuburg, 1502 - 1559
- Ottoboni, Pietro, Cardinal, 1667 - 1740
- Otway-Cave, Catherine, ca. 1811 - 1875
- Oudry, Jean-Baptiste, 1686 - 1755
- Ouless, Walter William, R.A., 1848 - 1933
- Ouradou, François Maurice, 1822 - 1884
- Oury, Charles, fl. ca. 1849-70?
- Outkine, Nicolai, 1780 - 1868
- Outreman, Henri d', 1546 - 1605
- Outrim, John, fl. 1834 - 1854
- Ovens, Pamela, 1903 - 1985
- Overbeck, Friedrich, 1789 - 1869
- Overbeek, Bonaventura van, ca. 1660 - 1706
- Overbeek, Michel van
- Overton, Henry, III, fl. 1751-65
- Overton, Henry, II, 1704 - 1751
- Overton, John, 1640 - 1713
- Ovid, 43 B.C. - 017 or 018 A.D.
- Owen, G. E. Elmslie, 1899 - 1964
- Owen, Richard, 1804 - 1892
- Owen, Samuel, 1768 - 1857
- Owen, Thomas, fl. ca. 1821 - 1831?
- Owen, William, fl. 1733-1793
- Owen, William, R.A., 1769 - 1825
- Oxford and Mortimer, Edward Harley, 2nd Earl of, 1689 - 1741
- Oxford and Mortimer, Robert Harley, 1st Earl of, 1661 - 1724
- Oxford University Press
- Oxford, Edward Harley, 5th Earl of, 1773 - 1849
- Ozanam, Jacques, 1640 - 1717
- Ozanne, Jeanne Françoise, 1735 - 1795
- Ozanne, Nicolas Marie, 1728 - 1811
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