Ottoman Empire, 1288-1918 (71)
Old English, ca. 450-1100 (66)
Old High German, 750-1050 (8)
Orange Revolution, 2004. (5)
Occupation, 1941-1944 (4)
Old Kingdom, ca. 2686-ca. 2181 B.C (4)
Occupation, 1939-1945 (3)
Opium War, 1840-1842 (3)
Ordovician (3)
October Crisis, 1970. (1)
Oligocene (1)
Otto I, 936-973 (1)
Overthrow of the Monarchy, 1893. (1)