In this paper we present a mesoscopic model of the transport and electrochemical processes inside a Lithium-O-2 battery cathode pore. The model dynamically resolves both Oxygen Reduction Reaction (ORR) thin film and solution phase mechanisms together with the transport of O-2, Li+ and LiO2 in the electrolyte. It is supported on an extension to three dimensions of our Kinetic Monte Carlo (KMC) Electrochemical Variable Step Size Method (E-VSSM) recently published by our group in [M. A. Quiroga and A. A. Franco, J. Electrochem. Soc., 162, E73 (2015)]. The model allows predicting porosity evolution as a function of multiple operational, physical and geometrical parameters including the pore size and inlet/ outlet channel size, O-2 and Li+ concentration, the property of the solvent as well as the applied overpotential. The investigation of the impact of these different aspects reveals that at the mesoscale level, the overall ORR kinetics and the discharge mechanism strongly depend on a balance between the geometrical features of the pore and the transport as well as the electrochemical properties of the system. (C) The Author(s) 2015 Published by ECS. All rights reserved.