We report the synthesis, characterization, and reactivity of [LFe3(PhPz)3OMn(sPhIO)][OTf]x (3: x = 2; 4: x = 3), where 4 is one of very few examples of iodosobenzene–metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, 57Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (FeIII2FeIIMnII vs. FeIII3MnII) influence oxygen atom transfer in tetranuclear Fe3Mn clusters. In particular, a one-electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two orders of magnitude.