The present work reports the oxidation of copper nanowires via microwave irradiation under wet conditions. Structural characterization of the nanowires has been carried out by X-ray diffraction, transmission electron microscopy and scanning electron microscopy coupled with focused ion beam, electron backscatter diffraction and energy dispersive X-ray spectroscopy. The copper nanowires were essentially monocrystalline with < 110 > axes, albeit growth twins have also been observed. Oxidation of the metal nanowires resulted in the growth of polyhedral Cu2O crystals arranged in twinned configurations around the nanowire axis. The crystallographic orientation of the twin variants is compatible with a cube-on-cube orientation relation originating from the epitaxial growth of Cu2O from pure Cu.