The Onega is a river in Kargopolsky, Plesetsky, and Onezhsky Districts of Arkhangelsk Oblast in Russia. The Onega connects Lake Lacha with the Onega Bay in the White Sea southwest of Arkhangelsk, flowing in the northern direction. The discharge at the source is 74.1 cubic metres per second (2,620 cu ft/s) and at the mouth is 505 cubic metres per second (17,800 cu ft/s). The river is 416 kilometres (258 mi) long, and the area of its basin 56,900 square kilometres (22,000 sq mi). Its main tributaries are the Voloshka (right), the Kena (left), the Mosha (right), the Kodina (right), and the Kozha (left). The major tributary of the Lake Lacha is the Svid.

Kargopolsky District is an administrative district (raion), one of the twenty-one in Arkhangelsk Oblast, Russia. As a municipal division, it is incorporated as Kargopolsky Municipal District. It is located in the southwest of the oblast and borders with Plesetsky District in the north, Nyandomsky District in the northeast, Konoshsky District in the east, Kirillovsky District of Vologda Oblast in the south, Vytegorsky District, also of Vologda Oblast, in the southwest, and with Pudozhsky District of the Republic of Karelia in the northwest. The area of the district is 10,127 square kilometers (3,910 sq mi). Its administrative center is the town of Kargopol. Population: 18,466 (2010 Russian census); 21,514 (2002 Census); 24,589 (1989 Soviet census). The population of Kargopol accounts for 55.3% of the district's total population.