Oster (Ukrainian: Осте́р [osˈtɛr]; Russian: Остёр, romanized: Ostjór) is a city located where the Oster River flows into the Desna, in Chernihiv Raion, Chernihiv Oblast of Ukraine. Oster hosts the administration of Oster urban hromada, one of the hromadas of Ukraine. Its population is 5,655 (2021 est.)Today Oster is a river port with a cotton-textile factory and a food industry. Some parts of the old fortress in Oster have been preserved, as have the remains of the Saint Michael's Church, constructed in 1098 and the only preserved church of the medieval principality of Pereyaslav.