Distillation column system for obtaining oxygen by cryogenic fractionation of air, comprising - a high-pressure column (101; 201) and a low-pressure column (102; 202), - a main condenser (103; 203) configured as a condenser-evaporator, the liquefaction space of the main condenser being in flow connection (104, 105, 114, 115; 204, 205, 214, 215) with the top of the high-pressure column, - and comprising an argon column (152; 252) which - is in flow connection with an intermediate point in the low-pressure column, and - has means (37, 163) of drawing off an argon-enriched stream and - an argon column top condenser (155, 255) which is configured as a condenser-evaporator and is in flow connection (37, 62, 41) with the top of the argon column (152, 252), - the low-pressure column has an upper mass transfer region, a lower mass transfer region and a middle mass transfer region, - the middle mass transfer region has at least one first mass transfer space which is open in the upward direction toward the upper mass transfer region and in the downward direction toward the lower mass transfer region, wherein - the upper mass transfer region has a liquid collector at its bottom end, - the first mass transfer space has a liquid distributor at its top, - the argon column top condenser (155, 255) is arranged within the low-pressure column between the upper and middle mass transfer regions, and in that - the argon column top condenser is configured as a forced-flow evaporator, having an evaporation space which has an inlet at its bottom end and an outlet at its top end, and the outlet being connected to the liquid distributor of the first mass transfer space, the system further including - means of introducing liquid from the liquid collector beneath the upper mass transfer region into the inlet of the evaporation space of the argon column top condenser, and - a vessel, a two-phase conduit which is connected to the outlet of the evaporation space of the argon condenser and to the inlet of the vessel, a gas conduit for drawing off gas from the vessel and containing a control valve, and a liquid conduit for introducing liquid from the vessel into the liquid distributor at the top of the middle mass transfer section. - a high-pressure column (101; 201) and a low-pressure column (102; 202), - a main condenser (103; 203) configured as a condenser-evaporator, the liquefaction space of the main condenser being in flow connection (104, 105, 114, 115; 204, 205, 214, 215) with the top of the high-pressure column, - and comprising an argon column (152; 252) which - is in flow connection with an intermediate point in the low-pressure column, and - has means (37, 163) of drawing off an argon-enriched stream and - an argon column top condenser (155, 255) which is configured as a condenser-evaporator and is in flow connection (37, 62, 41) with the top of the argon column (152, 252), - is in flow connection with an intermediate point in the low-pressure column, and - has means (37, 163) of drawing off an argon-enriched stream and - an argon column top condenser (155, 255) which is configured as a condenser-evaporator and is in flow connection (37, 62, 41) with the top of the argon column (152, 252), - the low-pressure column has an upper mass transfer region, a lower mass transfer region and a middle mass transfer region, - the middle mass transfer region has at least one first mass transfer space which is open in the upward direction toward the upper mass transfer region and in the downward direction toward the lower mass transfer region, wherein - the upper mass transfer region has a liquid collector at its bottom end, - the first mass transfer space has a liquid distributor at its top, - the argon column top condenser (155, 255) is arranged within the low-pressure column between the upper and middle mass transfer regions, and in that - the argon column top condenser is configured as a forced-flow evaporator, having an evaporation space which has an inlet at its bottom end and an outlet at its top end, and the outlet being connected to the liquid distributor of the first mass transfer space, the system further including - means of introducing liquid from the liquid collector beneath the upper mass transfer region into the inlet of the evaporation space of the argon column top condenser, and - a vessel, a two-phase conduit which is connected to the outlet of the evaporation space of the argon condenser and to the inlet of the vessel, a gas conduit for drawing off gas from the vessel and containing a control valve, and a liquid conduit for introducing liquid from the vessel into the liquid distributor at the top of the middle mass transfer section.