The electrocatalytic ammonia oxidation reaction (AOR) to molecular dinitrogen (N-2) is an essential component within a sustainable nitrogen cycle. The state-of-the-art Pt nanocatalyst, preferably terminated with (100) facets, suffers from a large overpotential (>0.5 V) and rapid deactivation, the origin of which remains largely unexplained due to the intrinsic complexity of solid-electrolyte interfaces. Within the framework of grand-canonical density functional theory (GC-DFT), we show that on Pt(100) the dehydrogenation of *NH2 is the potential determining step and that the *OH species, thermodynamically stable at >0.5 V vs RHE while overlooked in previous studies, plays an important role in kinetics by preferential stabilization of *NH via hydrogen bonding. Attributed to such favorable adsorbate-adsorbate interactions, *NH2 dehydrogenation is thermoneutral at 0.5 V vs RHE forming *NH species that can then dimerize easily at the 4-fold hollow sites, capturing the experimentally observed onset potential. At high operating potentials (>0.63 V vs RHE) where the *NH dehydrogenation to *N becomes thermodynamically feasible, surface deactivation occurs. However, the dimerization of *N with *N or *NH is kinetically facile, which suggests that the adsorbed *N is only the precursor to poisoning species, e.g., *NO, on Pt(100). The mechanistic insights obtained in this study could be exploited in new strategies of designing active, selective, and robust electrocatalysts for ammonia oxidation.