In this work, we have investigated the hyperthermal collisions of atomic oxygens with graphene through molecular dynamics simulations using the ReaxFF reactive force field. First, following Paci et al. (J. Phys. Chem. A 2009, 113, 4677-4685), 5-eV energetic collisions of atomic oxygen with a 24-atom pristine graphene sheet and a sheet with a single vacancy defect, both functionalized with oxygen atoms in the form of epoxides, were studied. We found that the removal of an O(2) molecule from the surface of the graphene sheet occurs predominantly through an Eley-Rideal-type reaction mechanism. Our results, in terms of the number of occurrences of various reactive events, compared well with those reported by Paci et al. Subsequently, energetic collisions of atomic oxygen with a 25-times-expanded pristine sheet were investigated. The steady-state oxygen coverage was found to be more than one atom per three surface carbon atoms. Under an oxygen impact, the graphene sheet was always found to buckle along its diagonal. In addition, the larger sheet exhibited trampoline-like behavior, as a result of which we observed a much larger number of inelastic scattering events than those reported by Paci et al. for the smaller system. Removal of O(2) from the larger sheet occurred strictly through an Eley-Rideal-type reaction. Investigation of the events leading to the breakup of a pristine unfunctionalized graphene sheet and the effects of the presence of a second layer beneath the graphene sheet in an AB arrangement was done through successive impacts with energetic oxygen atoms on the structures. Breakup of a graphene sheet was found to occur in two stages: epoxide formation, followed by the creation and growth of defects. Events leading to the breakup of a two-layer graphene stack included epoxide formation, transformation from an AB to an AA arrangement as a result of interlayer bonding, defect formation and expansion in the top layer, and finally erosion of the bottom layer. We observed that the breakup of the two-layer stack occurred through a sequential, layer-by-layer, erosion process.