In the existing air-separating unit (ASU) that carries out the cycles of medium pressure it is not possible to use effectively the operation of air expansion in the turbo gas-expansion machine. The scheme of ASU which includes the gas-expansion machine-compressor unit is considered. The mathematical models used in the course of the calculations enable us to find the optimum values of the direct stream pressure and the consumption of the processed air when they were changed. The optimum pressure and air consumption correspond to two extremums: a minimum of specific work and a maximum of oxygen production. It was found that when the pressure of the direct stream was reduced from 6,5 to 4,6 MPa and simultaneously the air consumption was increased the cost per unit of the liquid oxygen production decreased from 1,15 to 0,98 kWh/kg.