Functionalization by oxygen-containing groups, electronic band gap, and conductivity produced by the functionalization in the disordered metallic nitrogen-containing multi-wall carbon nanotubes (N-MWCNTs), are investigated experimentally and theoretically. Oxygen in the carboxyl groups is located in top sites at the nanotube surface and produces the bang gap. The energy gap is found to have a width of approximately 1.3 eV which does not depend on diameter, oxygen concentration, and structure of the groups. Oxygen localized in the bridge position in the hydroxyl group promotes the appearance of charge carriers in a gap. The charge carrier’s concentration and conductivity produced by oxygen groups are shown to be determined by the features of electron scattering. Theory connecting the functionalization, bang gap, and electronic properties of the nanotubes, is developed.