Ostashkov is a town and the administrative center of Ostashkovsky District in Tver Oblast, Russia, on a peninsula at the southern shore of Lake …Ostashkov is a town and the administrative center of Ostashkovsky District in Tver Oblast, Russia, on a peninsula at the southern shore of Lake Seliger, 199 kilometers west of Tver, the administrative center of the oblast. Population: 18,088 ; 20,660 ; 27,401.