Intensities of spin-forbidden transitions in electronic absorption and emission spectra of molecular oxygen are analyzed in order to understand the key mechanisms of spin-states mixing induced by spin-orbit coupling (SOC) and the ways to overcome spin prohibition for various photophysical and biochemical processes. Multireference configuration interaction calculations with SOC account are used to generalize spin-selection rules for the oxygen atmospheric and Herzberg bands in free O2 molecule and in collision complexes. Intensity enhancement of the atmospheric a1Δg → X3Σg−, b1Σg+ → X3Σg−, and Noxon b1Σg+ → a1Δg transitions upon bimolecular collisions are compared with those for Herzberg III transitions A′3Δu ← X3Σg−. Electric quadrupole, dipole, and magnetic approximations are used for transition probability calculations. Intensity distribution in rotational lines is also considered. With this background, we propose some simple spin-selection rules for dioxygen activation in enzymatic reactions. © 2013 Wiley Periodicals, Inc.