Orsk (ôrsk) [key], city (1989 pop. 271,000), Orenburg region, in the foothills of the S Ural Mts., central Russia, on the Ural River. It is the center of the Orsk-Khalilovo industrial area, which has rich iron, copper, nickel, and coal deposits. There are metallurgical and chemical plants, machine works, and a major oil refinery. Orsk was founded in 1735.