City (pop., 2001 est.: 1,138,400), southwestern Russia, at the confluence of the Irtysh and Om rivers.
Founded in 1716 as a military stronghold, Omsk became a city in 1804. It remained the headquarters of the Siberian Cossacks until the late 19th century. In 191819 it was the seat of the anti-Bolshevik government of Adm. Aleksandr Kolchak . Its growth was spurred by the building of the Trans-Siberian Railroad in the 1890s and the onset of World War II. Pipelines from the Volga-Urals and western Siberian oil fields supply the refinery, and the petrochemical industry is also is important.