A method for operating an oxygen reducing system assigned to an enclosed room which comprises a compressor system (4; 4.1, 4.2) for compressing an initial gas mixture containing at least nitrogen and oxygen and a gas separation system (3; 3.1, 3.2) downstream of the compressor system (4; 4.1, 4.2) for separating off at least a portion of the oxygen contained in the initial gas mixture and for providing a nitrogen-enriched gas at the outlet of the gas separation system (3; 3.1, 3.2) which can be fed to the enclosed room in order to set and/or maintain a reduced oxygen content, wherein the method comprises the following method steps: i) providing an initial gas mixture consisting of nitrogen, oxygen and other components as applicable; ii) compressing the provided initial gas mixture in the compressor system (4; 4.1, 4.2) of the oxygen reducing system; iii) feeding the compressed initial gas mixture to the gas separation system (3; 3.1, 3.2) of the oxygen reducing system and separating off at least a portion of the oxygen contained in the initial gas mixture in the gas separation system (3; 3.1, 3.2) such that a nitrogen-enriched gas is provided at the outlet of the gas separation system (3; 3.1, 3.2); and iv) introducing the nitrogen-enriched gas provided at the outlet of the gas separation system (3; 3.1, 3.2) into the enclosed room, characterized in that the oxygen reducing system is alternatively operated in a first operating mode, in which the oxygen reducing system provides preventive fire protection or firefighting in the enclosed room, or in a second operating mode, in which the airtightness of the enclosed room is determined and/or monitored, wherein the method further comprises the following method steps in the second operating mode of the oxygen reducing system: v) determining the pressure prevailing in the enclosed room or set in the enclosed room in relation to the ambient air pressure; vi) comparing at least one differential pressure value determined in method step v) to a corresponding reference value; and vii) evaluating the result of the comparison obtained in method step vi), wherein a conclusion can be drawn as to the airtightness of the enclosed room depending on the extent of deviation between the determined differential pressure value and the respective reference value. i) providing an initial gas mixture consisting of nitrogen, oxygen and other components as applicable; ii) compressing the provided initial gas mixture in the compressor system (4; 4.1, 4.2) of the oxygen reducing system; iii) feeding the compressed initial gas mixture to the gas separation system (3; 3.1, 3.2) of the oxygen reducing system and separating off at least a portion of the oxygen contained in the initial gas mixture in the gas separation system (3; 3.1, 3.2) such that a nitrogen-enriched gas is provided at the outlet of the gas separation system (3; 3.1, 3.2); and iv) introducing the nitrogen-enriched gas provided at the outlet of the gas separation system (3; 3.1, 3.2) into the enclosed room, the oxygen reducing system is alternatively operated in a first operating mode, in which the oxygen reducing system provides preventive fire protection or firefighting in the enclosed room, or in a second operating mode, in which the airtightness of the enclosed room is determined and/or monitored, wherein the method further comprises the following method steps in the second operating mode of the oxygen reducing system: v) determining the pressure prevailing in the enclosed room or set in the enclosed room in relation to the ambient air pressure; vi) comparing at least one differential pressure value determined in method step v) to a corresponding reference value; and vii) evaluating the result of the comparison obtained in method step vi), wherein a conclusion can be drawn as to the airtightness of the enclosed room depending on the extent of deviation between the determined differential pressure value and the respective reference value. v) determining the pressure prevailing in the enclosed room or set in the enclosed room in relation to the ambient air pressure; vi) comparing at least one differential pressure value determined in method step v) to a corresponding reference value; and vii) evaluating the result of the comparison obtained in method step vi), wherein a conclusion can be drawn as to the airtightness of the enclosed room depending on the extent of deviation between the determined differential pressure value and the respective reference value.