Oka (ō´kə), village, S Que., Canada, on the north shore of the Lake of the Two Mountains (a widening of the Ottawa) and SW of Montreal. It is noted as the site of a Trappist monastery and farm (est. 1881), where Oka cheese is made. An agricultural institute there is affiliated with the Univ. of Montreal. About 600 Native Americans live at Oka.

Oka (əkä´), river, c.925 mi (1,490 km) long, rising S of Orel, central European Russia. It flows N past Orel and Kaluga, E past Serpukhov, Kolomna, and Ryazan, and then NE past Murom to join the Volga River at Nizhny Novgorod. It is navigable by large vessels below Kolomna, c.550 mi (890 km) upstream, and traverses densely populated agricultural and industrial areas. Among its tributaries are the Moskva, the Klyazma, and the Moksha.

Oka, river, c.600 mi (970 km) long, rising in the Sayan Mts., Buryat Republic, S central Siberian Russia. It flows N through Irkutsk oblast to join the Angara River below Bratsk. The lower Oka valley is flooded by waters impounded behind Bratsk Dam.