Oka (Angara)The Oka is a river in Siberia, left tributary of the Angara. It is long, and has a drainage basin of.CourseIt originates in the Oka Lake, Oka Plateau, Sayan Mountains at the western end of Buryatia. It flows roughly northwestwards with the Bolshoy Sayan to the southwest. After it bends northeastwards the river flows between the Kropotkin Range and the Belskye Goltsy. After the Khoyto-Oka tributary joins it from the left it enters Irkutsk Oblast and flows in a roughly more northern direction. Finally the Oka discharges into the Bratsk reservoir.