One of the key challenges in environmental catalysis is the remediation of NO_x (x=1,2) from fuel combustion emissions. NO oxidation to NO₂ is an important step in this process. The reaction is effectively promoted by Pt under high pressures of O₂. These high pressures generate high coverages of O at the Pt surface, which have marked affects on the thermodynamics and kinetics of processes in the NO oxidation mechanism. In this work we investigate the thermodynamics and kinetics of NO oxidation on Pt(111) at O coverages resulting from typical reaction conditions. We compare different reaction pathways and predict a mechanism for Pt-catalyzed NO oxidation. From this we estimate which steps control the reaction rate. Understanding how Pt functions will lead to design of better catalysts for NO oxidation and may facilitate rational catalyst design for other transition metal-catalyzed oxidations as well.