A two-band model of superconductivity which incorporates both the intraband and interband pairing terms has been reexamined, incorporating the attractive free carrier negative U-centre interacting mechanism in addition to the attractive electron-phonon interaction, to investigate the isotope effect of high-Tc superconductors. Numerical calculations show that the model can be used to interpret the behaviour of the isotope effect of the oxide superconductors in quantitative agreement with experimental data. Some parameters of the model are predicted for high-Tc superconductors.