Oka (Russian: Ока́, IPA: [ɐˈka]) is a river in central Russia, the largest left tributary of the Volga. It flows through the regions of Oryol, Tula, Kaluga, Moscow, Ryazan, Vladimir, and Nizhny Novgorod and is navigable over a large part of its total length, as far upstream as to the town of Kaluga. Its length exceeds 1,500 kilometres (930 mi).