The Oka (Russian: Ока, Buryat: Аха, romanized: Akha) is a river in Siberia, left tributary of the Angara.

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.

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