Ab initio density functional theory (DFT) calculations of the interactions between isolated infinitely-long semiconducting zig-zag (10, 0) or isolated infinitely-long metallic arm-chair (5, 5) single-walled carbon-nanotubes (SWCNTs) and single oxygen-molecules are carried out in order to determine the character of molecular-oxygen adsorption and its effect on electronic transport properties of these SWCNTs. A Green's function method combined with a nearest-neighbor tight-binding Hamiltonian in a non-orthogonal basis is used to compute the electrical conductance of SWCNTs and its dependence on the presence of topological defects in SWCNTs and of molecular-oxygen adsorbates. The computational results obtained show that in both semiconducting and metallic SWCNTs, oxygen-molecules are physisorbed to the defect-free nanotube walls, but when such walls contain topological defects, oxygen-molecules become strongly chemisorbed. In semiconducting (10, 0) SWCNTs, physisorbed O-2-molecules are found to significantly increase electrical conductance while the effect of 7-5-5-7 defects is practically annulled by chemisorbed O-2-molecules. In metallic (5, 5) SWCNTs, both O-2 adsorbates and 7-5-5-7 defects are found to have a relatively small effect on electrical conductance of these nanotubes. (C) 2003 Elsevier Science B.V. All rights reserved.