Omsk city and administrative centre of Omsk oblast (region), west-central Russia, on the Irtysh River at its junction with the Om. Omsk, founded in 1716 as a stronghold at the eastern end of the Ishim fortified line between the Tobol and the Irtysh, developed as an agricultural centre and became a city in 1804. Its military function as headquarters of the Siberian Cossacks lasted until the late 19th century. In 1918–19 it was the seat of the anti-Bolshevik government of Adm. A.V. Kolchak