A kinetic Monte Carlo surface reaction lattice gas model is investigated for CO oxidation under sub-atmospheric conditions. The present Letter incorporates recent experimental observations suggesting the formation of surface oxide phase with higher catalytic activity than chemisorbed oxygen. Rate oscillations occur due to collective oxidation of the adsorbed oxygen sites as soon as their coverage reaches a critical threshold. While the chemisorbed oxygen follows the Langmuir-Hinshelwood (LH) mechanism, with the reaction taking place between adsorbed neighboring CO and O, the surface oxide can also directly react with the gas phase CO molecule. (C) 2012 Elsevier B.V. All rights reserved.