Iron- and nitrogen-doped carbon (Fe-N-C) materials have been suggested as the most promising replacement for Pt-based catalysts in oxygen reduction reaction (ORR) owing to the FeN₄ active moiety. Based on the relationship between the oxygen binding energy and the catalytic activity, Fe-N-C has very strong oxygen binding energy so that hard to desorb the final reaction intermediate of *OH. Herein, we provide for the first time an effective method of tuning the active moiety using a phosphine gas-phase treatment for Fe-N-C. Combined analyses of experimental and computational results reveal that the conventional FeN₄ moiety is transformed into FeN₃PO through the P-doping post-treatment. Furthermore, we propose an ORR mechanism on the unique FeN₃PO moiety based on a microkinetic model, in which *OH intermediates are considered. The invasive P-doping effects on the ORR performance are also validated in both anion exchange membrane fuel cell (AEMFC) and proton exchange membrane fuel cell (PEMFC).