The role of B 3Σu--2 3Σu+ spin-orbit mixing in the O 2 Schumann-Runge predissociation is investigated. The 2 3Σu+ state is found to cross the B 3Σu- state near 2.0 Å with an interaction matrix element of approximately 55 cm -1. This state contributes to the widths of the Bv ≥ 6 levels, but introduces only small level shift perturbations. When the partial widths due to the 3Σu-- 3Σu+ interaction are added to the previously calculated widths due to the 5Π u, 3Π u, and 1Π u states, reasonable agreement is obtained with experimental measurements on O 16O 16 and O 18O 18. The possibility of non-Lorentzian line profiles and the dependence of the width on rotational quantum number is investigated. The approximation of the spin-orbit matrix element by its value at the crossing point is shown to be a good approximation for calculating the second difference perturbations.