Rate constants have been calculated using quasi-classical trajectories (QCT) for the vibrational relaxation of highly excited O-2: O-2(X (3)Sigma (-)(g), nu (i)) + O-2(X (3)Sigma (-)(g), 0) --> O-2(X (3)Sigma (-)(g), nu (f)) + O-2(X (3)Sigma (-)(g)), with 22 less than or equal to nu (i) less than or equal to 28. The present full-dimensional QCT results agree very well with recent quantum reduced dimensionality calculations, giving further support to the hypothesis that the observed experimental jump in depletion rates would be due partially to enhanced vibrational relaxation.