Oka is a great river in Russia, the biggest right confluent of the Volga. It flows through the Oblasts of Oryol, Tula, Kaluga, Moscow, Ryazan, Vladimir and Nizhny Novgorod. Its length exceeds 1500 km.

The name of Oka is cognate to aqua, the Latin word for water, and to the name of the Aa River in Switzerland. Historically, the river gave its name to the Upper Oka Principalities, situated upstream from Tarusa. One of the largest Russian cities, Nizhny Novgorod, was founded to protect the Oka's confluence with the Volga. The Russian capital, Moscow, sits on one of the Oka's tributaries - the Moskva River.

The link address is: http://en.wikipedia.org/wiki/Oka_River