This study investigates the reaction between silicon atom in its ground electronic state and ground-state molecular oxygen. The potential energy surfaces for the two competing reactions Si(3P) + O2(3Σg-) = SiO2(1Σg+) vs Si(3P) + O2(3Σg-) = SiO(1Σg+) + O(3P) are analyzed and compared. The lowest energy potential energy surface (PES) for each multiplicity is investigated within Cs symmetry. The entire potential energy surfaces were described using the multi-configuration self-consistent field (MCSCF) level of theory, augmented by multireference second order perturbation theory (MRMP2). Singles and doubles coupled cluster theory with perturbative triples, CCSD(T), energy calculations were also done at the MCSCF geometries. It is shown that the singlet reaction is thermodynamically favored, that the singlet product, SiO2 (1Σ g+), is the global minimum, and that both reactions have no net barrier. Extrapolation of the CCSD(T)/cc-pVTZ reaction enthalpies to the complete basis set (CBS) limit brings the calculations into excellent agreement with experimental data.