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.

The Volga is the longest river in Europe. Situated in Russia, it flows through Central Russia to Southern Russia and into the Caspian Sea. The Volga has a length of 3,531 km (2,194 mi), and a catchment area of 1,360,000 km2 (530,000 sq mi). It is also Europe's largest river in terms of average discharge at delta – between 8,000 m3/s (280,000 cu ft/s) and 8,500 m3/s (300,000 cu ft/s) – and of drainage basin. It is widely regarded as the national river of Russia. The hypothetical old Russian state, the Rus' Khaganate, arose along the Volga c. 830 AD. Historically, the river served as an important meeting place of various Eurasian civilizations.