The Oka is a river in central Russia, the largest right 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 the town of Kaluga. Its length is 1,500 km (930 mi) and its catchment area 245,000 km2 (95,000 sq mi). The Russian capital Moscow sits on one of the Oka's tributaries—the Moskva.

Nizhny Novgorod Oblast is a federal subject of Russia. Its administrative center is the city of Nizhny Novgorod. It has a population of 3,119,115 as of the 2021 Census. From 1932 to 1990 it was known as Gorky Oblast.