Single-domain epitaxial ZnO films are grown near-room temperature (below 40 °C) on (0 0 0 1)-sapphire using reactive magnetron sputtering of a zinc target in the transition between the metallic and compound sputtering regimes. As the oxygen content in the reactive gas mixture is increased, the in-plane six-fold symmetry of hexagonal wurtzite ZnO, probed by φ -scan measurements, develops. Transmission electron microscopy analyses confirm that single-domain epitaxial layers are formed. This is accompanied by the incorporation of oxygen interstitial defects associated with oxygen over-stoichiometry and by compressive stresses. A model is proposed to explain the observed behaviour based on the transformation of the kinetic energy of fast oxygen particles into the mobility of the adatoms.