An iron fiber optics catalytic probe has been constructed and applied for the real-time measuring of the O-atom density in an Ar/O-2 afterglow. The recombination coefficient for the heterogeneous surface recombination of O atoms on the oxidized iron foil was measured at different temperatures between 400 and 950 K. The coefficient was found to be constant in the entire range of experimental conditions and had a value of 0.41 +/- 0.12. The iron fiber optics catalytic probe has an advantage over the classical nickel fiber optics catalytic probe: the probe signal is higher for the iron probe due to a higher recombination coefficient thus causing an easier real-time monitoring of the O-atom density. The O-atom density was measured in an afterglow of microwave plasma created at different discharge powers up to 300 W, at a constant Ar flow rate of 1000 sccm/min and at different oxygen flow rates between 50 and 300 sccm/min. The O-atom density was found to be dependent on oxygen flow. At low oxygen flow rates up to 100 sccm/min, a saturation of the O-atom density was obtained at a certain discharge power, while at high oxygen flow rate the O-atom density kept increasing with the increasing power. The results were explained by gas phase and surface phenomena.