Oka (Angara)
The Oka (Russian: Ока, Buryat: Аха, romanized: Akha) is a river in Siberia, left tributary of the Angara. It is 630 kilometres (390 mi) long, and has a drainage basin of 34,000 square kilometres (13,000 sq mi).
Course
It 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 (Бельские Гольцы). 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.