Oka. 1. A river in European Russia, which, rising near Orel, flows through the richest districts of central Russia, and joins the Volga nt Nijni-Novgorod, after a course of 840 miles. It has a basin of 127,000 square miles. 2. A river of Asiatic Russia, rising on the Chinese frontier and flowing north, through the government of Irkutsk, till it unites with the Angara at Bratskoi. It is from 400 to 500 miles in length, and receives the waters of the Ija from the west.