The chemisorption of singlet O-2 on the side wall of a series of zigzag (n,O) Single Walled Carbon Nanotubes (SWNTs) (n = 6 similar to 15) have been investigated by planewave/pseudopotential- based density functional theory. The process involves two steps, cycloaddition, followed by the breaking of an O-O bond with the formation of epoxy structures. The most stable adsorbate configurations have been determined for both steps. The results indicate that the saturation of a zigzag C-C bond induces less stress on a carbon nanotube than the saturation of an axial C-C bond. The formation of the epoxy structure is exothermic and the degradation of carbon nanotubes upon air exposure is possible for tubes with a diameter below 1 nm. Overall, the adsorption energy decreases with increasing tube diameter, and the electronic structure also affects the reactivity. Furthermore, the changes in electronic structures introduced by the adsorption of oxygen have also been examined.