Collisions between SO2 and O are investigated through Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) calculations, for SO2 internal energies and relative collision velocities that are of interests to the atmospheric dynamics study of Io. Three possible dissociation reactions are modeled, and reaction cross sections, σr,MD, and the equivalent variable hard sphere (VHS) cross sections, σVHS,MD, are obtained based on the MD/QCT results. The ratio of σr,MD to σVHS,MD indicate that in high collision velocity regime the dissociation reaction is the dominant process. Reaction probabilities are further obtained by using the sum of the reaction cross section and equivalent VHS cross section as the total collision cross section. This method provides the consistent total collision cross sections and reaction probabilities that can be applied in direct simulation Monte Carlo (DSMC) for large range of total collisional energies whereas the VHS cross section and the reaction probability from the total collision energy (TCE) model cannot be applied in high collision velocity regime that needs to be modeled.