The heme monooxygenase nitric oxide synthase (NOS) catalyzes the degradation reaction of L-arginine to L-citrulline, where NO(g) is concomitantly produced as a side product. This reaction is a two-step process and the exact mechanistic details pertaining to the second step, where N-hydroxy-L-arginine (NHA) is oxidized to L-citrulline and NO(g) remain elusive to this day. Interestingly, a heme peroxo (heme-PO) intermediate has been implicated as the active oxidant therein, where the two peroxo oxygen atoms ultimately get incorporated into NHA leading to L-citrulline and NO(g). In heme enzymes, proximal axial ligands have been shown to tune the reactivities of high-valent iron intermediates, as supported by exhaustive enzymatic and model studies. However, the effects of axial ligation in dictating the reactivities of heme-PO intermediates within physiological systems have not been fully evaluated. This work presents a functional model system employing oxime substrates as structural mimics of NHA to reproduce the second mechanistic step of nitric oxide synthase. Furthermore, various synthetic analogs were utilized to model the role(s) of proximally coordinating amino acid of heme enzymes. Specifically, imidazole and pyridine moieties served as mimics for histidine, phenolate functionalities modeled tyrosine, and thiolate/thiophenolate analogs were utilized to mimic the cysteine coordination environment in NOS. Our findings reveal that axial ligation of heme-PO adducts escalates the rates of their nucleophilic reactivity, wherein the anionic ligands exhibited the most pronounced “push effect”. Coordination of these axial ligands are accompanied by distinct geometric and electronic perturbations, which were investigated with an array of spectroscopic methods under cryogenic conditions such as UV-visible (UV-vis), electron paramagnetic resonance (EPR), 2H nuclear magnetic resonance (NMR), resonance Raman (rR) as well as by complementary theoretical studies. Experimentally computed activation parameters unequivocally indicate that axial ligation leads to the modification of activation parameters pertaining to reactivities between heme-PO adducts and oxime substrates. All reaction products, including the final ketone as well as NO–, have been fully characterized.