Potential energy curves of triplet states for collinear O-O-2 were calculated by ab initio CASSCF and MRSDCI methods. We found a pseudocrossing between 1 (3)Sigma(-) (6 (3)A(')) and 2 (3)Sigma(-) (7 (3)A(')) states at long O-O-2 separation. The electronic quenching reaction, O(D-1)+O-2(X (3)Sigma(g)(-))-->O(P-3)+O-2(b (1)Sigma(g)(+)), is dominated by the nonadiabatic transition via the pseudocrossing. The collision energy dependence of the quenching reaction probability, which is evaluated by Zhu and Nakamura's formula, is found to be in good agreement with experiment.