We investigated the effect of atomic and molecular oxygen adsorption on the geometries and electronic properties of small and large armchair single-walled carbon nanotubes (SWCNTs) by means of the density functional theory (DFT). We Calculated the equilibrium geometries, energetics, and electronic properties of the nanotubes as well as the tube-molecules. Global indices Such as electronic chemical potential and hardness Were calculated using the Kopmann's theorem. Our investigation involved the physisorption of molecular oxygen, chemisorption of atomic oxygen, and formation of epoxide-like structures. The adsorption energies of the oxygen molecules physisorbed to different sites were determined by calculating the short-range interactions. The effect of the tube diameter on the stability of the tube-O-2 system was studied for the different sites. Also we considered the orientation Of O-2 molecule during adsorption of O-2 molecule on the outer surface of tubes. Adsorption of oxygen atom on top of the carbon atom of the tubes was also considered. We found out that O atoms bind to the Outer surface of the SWCNTs to give stable epoxide-like Structures. The most stable epoxide-like structure on the outer surface of the nanotubes was the (4,4)-O system with a calculated adsorption energy of -2.944 eV. (C) 2009 Elsevier Inc. All rights reserved.