Onega, Lake, a lake in Russia, about 180 miles (290 km) northeast of St. Petersburg. With an area of some 3,800 square miles (9,800 km2), it is the second-largest lake in Europe, after Lake Ladoga. Lake Onega is a link in a canal and waterway system that connects the White Sea, the Gulf of Finland, and the Volga River. The lake is used also for commercial fishing.