The reduction of graphene oxide surfaces yielding molecular CO/CO2 is studied from first principles using density functional theory. We find that this reaction can proceed exothermically only from surface precursors containing more oxygen atoms than strictly needed to produce CO/CO2 in the gas phase. The calculations show that the lowest-energy configurations of multiple O adsorbates do not involve clustering of epoxy groups (the stable form of O adatoms on graphitic surfaces) but always contain lactone groups either in lactone−ether or in ether−lactone−ether form. We identify these lowest-energy structures as the main reaction precursors. The O adatoms near the lactone group catalyze its gasification to CO/CO2 by reducing the activation energy from above 1.8 eV (from an isolated lactone) to below 0.6 eV (from a lactone−ether). In addition, the residual O adatoms left behind after the lactone gasification minimize the energy of the graphitic products by saturating the dangling bonds of the resulting defective surface. By analyzing defect-free as well as defective surfaces, we identify a common set of concerted reaction mechanisms in which the formation of the gas products and the saturation of the newly formed C vacancies happen simultaneously. The calculated activation energies are in good agreement with the available experimental data.