Beam-surface scattering experiments and theoretical direct dynamics based on density functional theory calculations are used to investigate hyperthennal collisions between O(P-3) and highly oriented pyrolytic graphite (HOPG). The simulations suggest that the HOPG surface becomes functionalized with epoxide groups. Intersystem crossing (ISC) between the lowest-energy triplet and singlet potenttial-energy surfaces is not necessary for this functionalization to occur. Both theory and experiment indicate that incoming O atoms can react at the surface to form O-2 by way of an Eley-Rideal mechanism. They also suggest that the collisions can result in the production of CO and CO2 by way of both direct and complex reaction mechanisms. The direct dynamics simulations provide significant insight into the details of the complex reaction mechanisms. Serniquinones are present at defect sites and can form in functionalized pristine sheets, the latter resulting in the fort-nation of a defect. Direct collision of an incoming O atom with a serniquinone or vibrational excitation caused by a nearby O-atom collision can cause the release of the serniquinone CO, forming carbon monoxide. The CO may react with an oxygen atom on the surface to become CO2 before receding from the surface. The simulations also illustrate how epoxide groups neighboring semiquinones catalyze the release of CO. Throughout, the experimental results are observed to be consistent with the theoretical calculations.