An oxidation furnace for the oxidative treatment of fibers, in particular for producing carbon fibers, comprises a housing (12) which is gas-tight, apart from passage openings (18, 20, 70) for the fibers (22), inter alia. A process chamber (28) is located in the interior (14) of the housing (12). Deflecting rollers (34) guide the fibers (22) through the process chamber (28) in a serpentine manner in such a way that the fibers lie next to one another as a fiber carpet (22a), and the fiber carpet (22a) spans a plane between opposite deflecting rollers (34). An atmosphere-generating device (40) can generate a hot working atmosphere (38) and comprises a blowing device (42) with at least one outlet window (54) through which a hot working atmosphere can be blown into the process chamber (28) between two adjacent planes of the fiber carpet (22a). The working atmosphere (38) is guided into the process chamber (28) by a flow guiding system (50). The flow guiding system (50) comprises exchangeable flow guiding elements (60) with flow passages (62) which can be detachably and/or movably mounted on the blowing device (42), before the outlet window (54).