A system for reducing oxygen in a target room, particularly for the purpose of controlling or preventing fire, wherein the system comprises an enclosed buffer space which is fluidly connectable or connected to the target room for the as-needed introduction of at least a portion of the room air of the buffer space into the target room. The system moreover comprises an oxygen-reducing mechanism allocated to the buffer space for setting and maintaining a reduced oxygen content in the spatial atmosphere of the buffer space compared to the normal earth atmosphere such that the oxygen content in the spatial atmosphere of the buffer space is lower than the oxygen content in the spatial atmosphere of the target room. The system moreover comprises a mechanism for introducing room air from the buffer space into the target room as needed.