The structural and electronic properties of oxygen molecular adsorbed on the exterior surface of pristine and N-C or B-C defected (10,0) or (6,6) SiCNT have been investigated systematically using the first-principles projector-augmented wave potential within the density-functional theory under the generalized-gradient approximation. We find that for both pristine tubes the preferred adsorption sites of the O-2 molecule are above and nearly parallel to armchair Si-C bond whether physisorption or chemisorption. The strong chemical interaction between O-2 molecule and tube leads to not only a vanishing in magnetism of the O-2 molecule but also an outward relaxation of the underlying Si-C bond. The C atom substituted by N or B atom assists O-2 molecule adsorption above and nearly parallel to zigzag Si-N or Si-B bond as well as imparts a metallic character on the SiCNTs with higher concentration of the defects or a magnetism on the SiCNTs with lower concentration of the defects. Therefore, a combination of N or B doping followed by exposure to air may be an effective way to tune the electronic properties of the semiconducting SiCNTs. Furthermore, the lower binding energies for the pair of oxygen interstitials chemisorbed on N-C or B-C defected (10,0) or (6,6) SiCNT show that the oxygen molecule will dissociate to the pair of oxygen interstitials at the sidewall of N-C or B-C defected SiCNTs. (C) 2010 Elsevier B. V. All rights reserved.