We report a study of the ab initio quantum chemical calculations for an ion-molecule reaction in the N-2(+) + O-2 system at the CCSD(T)/6-31G*//MP2(full)/6-31G* level augmented by multireference configuration interaction (MRCI) calculations. For the charge transfer (CT) reaction N-2(+) + O-2 --> N-2 + O-2(+), different mechanisms of electron transfer exist according to the electronic state of N-2(+). Along the potential energy curve with C-2v symmetry, electron transfer to N-2(+)(X(2)Sigma(g)(+)) occurs via a quartet T-shaped intermediate complex where positive charge has already been distributed on the O-2 fragment, and the CT leads to O-2(+)(a(4)Pi(u)) with large endothermicity. In contrast, electron transfer to N-2(+)(A(2)Pi(u)) occurs via a doublet T-shaped intermediate complex, and the CT leads to O-2(+)(X(2)Pi(g)) with large exothermicity. The doublet and quartet energy surfaces that connect the reactant systems, the intermediate complexes, and the product systems are examined in detail. The quartet CT reaction path contributes to the vibrational deactivation of N-2(+)(X(2)Sigma(g)(+)). Related characteristics of the reaction dynamics are also discussed.