The reactions of oxygen radical cation O+ with haloalkanes are of importance in gas phase ion chemistry, but the studies on reactive patterns and dynamics are quite limited. In this work, the direct dynamics simulations have been carried out at the B3LYP/ECP/d level of theory for 4O+ + CH3I reaction at Ecoll = 4.5 eV, and the results are compared with the available experiments. The properties of stationary points on the potential energy surface for 4O+ + CH3I reaction have been characterized and supplemented in terms of our simulations. Variety of atomic reaction mechanisms are revealed and are found to be energetically accessible at high collision energy, including charge transfer, dissociative charge transfer, proton transfer, substitution, and complex rearrangement, which exclusively proceed by direct mechanism due to the high collision energy. Our work has confirmed the existence of the charge transfer product CH3I+, which is highly exothermic. The major product CH3+ can be produced in concert with IO molecule, besides the dissociative charge transfer process of CH3I+. Similarly, including the possible dissociation of CH3I+, product I+ can be formed by substitution reaction. Some trace products, i.e., CH2I+, H2CO+, and CH3O+, are also found in our simulations. This work is expected to enrich our understanding of the significant atmospheric concentrations of these methyl halides and their anthropogenic origins.