We consider oxygen interactions with realistic silica surfaces, including both experimentally observed nondefective surface reconstructions and experimentally observed surface defects. Nondefective models include clusters representing the site above a fully coordinated surface Si atom and bridging O atoms, and the defective models include clusters representing an under-coordinated Si defect, a nonbridging O defect, and a ring structure. Energies were obtained for the approach of atomic and molecular oxygen to these clusters in various configurations by using explicitly correlated CCSD(T)-F12b electronic structure theory and the Minnesota density functionals, which were found to be in good agreement. The Minnesota functionals were employed in binding energy calculations for all of the clusters, considering the singlet and triplet spin states for nondefective clusters and doublet and quartet spin states for defective clusters. We find that the chosen defects are energetically favorable sites for binding. The density functional energies were used to extend the empirical ReaxFFSiO potential for silica, which was previously parametrized for bulk silica polymorphs (van Duin et al. J. Phys. Chem. A, 2003, 107, 3803-3811), to model the gas-surface interactions represented by the defective and nondefective clusters presented here. Interaction energy predictions from ReaxFFSiOGSI agree very well with the density functional energies. ReaxFFSiOGSI can now be employed in reactive large-scale molecular dynamics simulations involving oxygen-silica gas-surface interactions such as oxide growth and the heterogeneous recombination of oxygen occurring on real silica surfaces.