The origin of the intensity of the B (3)Sigma(u)(-) <-- b (1)Sigma(g)(+) transition of molecular oxygen, first observed recently by Eppink et al. [J. Chem. Phys. 108, 1305 (1998)], is discussed. It is shown that the B <-- b transition borrows its intensity principally from the dipole-allowed B <-- X transition, through spin-orbit mixing between the X (3)Sigma(g)(-) and b (1)Sigma(g)(+) states. Estimated continuum photoabsorption cross sections and discrete oscillator strengths for the B <-- b system are presented.