- Oake, Henry Alfred (c1857-1936)
- Oakeley, Edward Murray (1840-1927)
- Oakeley, Sir Herbert Stanley (1830-1903)
- Oakes, Colin
- Oakes, Harry Noel (1885-)
- Oakes Ltd, John
- Oates, Ernest Henry (1891-1970)
- Oates, Vincent (1887-1945)
- Obbes, Joan Francois Nicholas (1825-1898)
- Oberdoerffer, Fritz (1895-1979)
- Oberg, Dr Paul Matthews (1904-)
- Oberhoffer, Heinrich (1824-1885)
- Obermüller, Karola (1977-)
- Obersold, H.
- Oberson, René (1945-)
- Obrecht, Jacob (1451-1505)
- O’Brien, Rev. Archibald (Archy) Clarence (1899-)
- O’Brien, Dr Charles Henry Fizgerald (1882-)
- O’Brien, Eugene (1945-)
- O’Brien, Grace (1887-)
- O’Brien, Vincent (1870-1948)
- Očenáš, Andrej (1911-1995)
- Ochs, Traugott (1854-1919)
- Ochs, Volker (1929-)
- Ochse, Dr Orpha Caroline (1925-)
- Ochsenkuhn, Sebastian (1521-1574)
- Öchsler, Elias (1850-1917)
- Ockeghem, Jan de (c1430-1495)
- Ockleston-Lippa, Kate (c1855-)
- O’Connell, Geoffrey
- O’Connell, Kevin (1958-)
- O’Connor, Leonard Fergus (1913-)
- O’Connor, Leonard Morton S. (1868-)
- O’Connor-Morris, Geoffrey (1886-1964)
- Ocvirk, Ivan (1883-1950)
- Odăgescu-Ţuţutianu, Irina (1937-)
- Odak, Krsto (1888-1965)
- Ødegaard, Henrik (1955-)
- O’Dell, Ernest
- Odell, Frank Walter (1893-)
- Odell, J. H.
- Odell & Co.
- O’Donnell, James
- O’Donnell, John (1948-)
- O’Donoghue, William Power
- Odoyevsky, Prince Vladimir Fyodorovich (1803-1869)
- Odstrčil, Karel (1930-1997)
- O’Dwyer, Robert (1862-1949)
- Oechsler, Elias (1850-1917)
- Oechsner, André Jean Laurent (1815-1886)
- Oehring, Helmut (1961-)
- Oers, J. C. van
- Oertzen, Rudolf von (1910-1990)
- Oesten, Max O. (1843-1917)
- Oetiker, August (1874-1963)
- Oetting, William H. (1875-1969)
- Oeveren, Johan van (1949-)
- Ofenbauer, Christian (1961-)
- O’Feely, Thomas George
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- Ogasapian, John K. (1940-2005)
- Ogbourne, F. C. M.
- Ogden, John W.
- Ogden, Nigel (1954-)
- Ogé, Joseph (1852-1940)
- Ogiermann, Christoph (1967-)
- Ogilvy, Alfred Walter (1877-)
- Ogilvy, Frederick Allsworth (1882-1957)
- Ogurkowski, B.
- O’Hara, Margaret Rose (1903-)
- O’Henry, Edward
- Ohlhanns, Franz (1861-1910)
- Ohlson, Ernest Frederick (1884-)
- Ohlson, Louis Curtin (1874-)
- Ohlsson, Sven Olof Anders (1932-)
- Ohnaka, Toraji (1896-1982)
- O’Hogan, Roger Matthew (1917-)
- Ohrmann, Fritz (1884-1935)
- Ohrmann, Jonathan (1747-1803)
- Ohrmann & Co
- Ohrmann & Nutt
- Ohse, Rheinhard (1930-)
- Okasaka, Keiki (1940-)
- Okazaki, Mitsuharu (1935-)
- Oke, Hubert George (1873-)
- Okeghem, Joannes (Jean de) (c1430-1495)
- Oker, John
- Okolo-Kulaks, Alexandre (1906-1989)
- Olagué, Bartolomeo Garcia de (-1658)
- Olagué, Frey Martinho Gracia de
- Olaizola Gabarain, José de (1883-1969)
- Oldaker, Angus Ian (1910-)
- Olbers, J. H.
- Olcese, Luigi
- Olczak, Krzysztof (1956-)
- Oldberg, Arne (1874-1962)
- Oldham, Edith Annie (1895-)
- Oldham, Kevin (1960-1993)
- Olding, John Herbert (c1856-)
- Oldknow, Alfred (1848-1912)
- Oldroyd, Dr George (1886-1951)
- O’Leary, Martin (1963-)
- Oley, H. Maxwell
- Oley, Johann Christoph (1738-1789)
- Olivares, Francisco (1787-)
- Olive, Edmund (-1824)
- Olive, Vivienne (1950-)
- Oliveira, João Pedro (1959-)
- Oliveira, dom Placiado Guimarães de (1886-1964)
- Oliver, Curt (1943-)
- Oliver, Darren
- Oliver, G. P.
- Oliver, Madra Emogene (1905-)
- Oliver, Stephen Michael Harding (1950-1992)
- Oliver Pina, Angel (1937-)
- Olives, Tóma
- Olivier, Gerrit Cornelius (1945-)
- Olkuśnik, Joachim (1927-2008)
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Organist 
Organist & composer 
Composer 
Organ builder 
Other
