Thermal oxidation is performed in a copper-contaminated ambient on Czochralski and float-zone silicon, and on samples with damaged and undamaged surfaces, in order to evaluate the role of oxygen supersaturation in the starting material and the influence of formation of oxidation-induced stacking faults on the microstructure and electrical properties of the Si/SiO2 interface underlayers. The microstructure is controlled using transmission electron microscopy and secondary ion mass spectrometry, and the minority carrier diffusion length is analyzed by the electron beam-induced current technique. It is shown that the thermal oxidation of Czochralski silicon induces the precipitation of large copper colonies associated with oxidation stacking faults decorated with oxygen at the Si/SiO2 interface. In float-zone silicon the oxidation stacking faults are themselves nucleation sites of copper precipitates. When there is no oxidation stacking fault formation (undamaged initial surface), copper precipitation may occur on dislocation nets. The extended defects are highly recombinant when they are associated to copper precipitates. The diffusion length is decreased in regions free of copper colonies-which indicates the growth of point-like defects during the thermal process-and partially restored by hydrogenation treatment.