The potential energy surfaces (PESs) for several electronic states involved in the reaction O+(S-4) + N-2( X(1)Sigma (+)) --> NO+(X(1)Sigma (+), nu') + N(S-4) and the role of the ionic N2O+ intermediate have been investigated by ab initio calculations. The (4)A " PES, which correlates with the ground state educts, has a barrier of about 1 eV, and therefore at low collision energies the reaction cannot take place adiabatically on this surface. However, the spin-orbit coupling in the entrance channel allows the system to pass into the Renner-Teller system of the X(2)Pi electronic ground state of the N2O+ intermediate. The reaction then proceeds on these surfaces up to the region in the exit channel where a similar coupling allows it to reach the product quartet asymptote. At collision energies higher than about 1 eV, the reaction proceeds mainly on the adiabatic PES of the (4)A " state. The A(2)Sigma (+) state of N2O+ predissociates via a vibronic coupling with the B(2)Pi state, and in bent structures via a spin-orbit coupling with the (4)A " component of the (4)Pi state. The electronic structure of the B(2)Pi state is found to be of crucial importance for the understanding of the reactive processes in low lying electronic states of N2O+.