Thin polycrystalline ZnO films are deposited onto Si (100) substrates by means of DC reactive sputter deposition from a Zn target in an argon–oxygen mixture. The influences of the oxygen content in the mixture and of the total gas pressure in the reactor during deposition as well as of applying postgrowth annealing in an oxygen flow at elevated temperatures on the structural, chemical, and electrical properties of the obtained films are discussed. Three types of thin-film structures were obtained: a porous polycrystalline Zn layer, a porous polycrystalline ZnO layer and a dense ZnO film. The porous Zn layer grew at oxygen content less than 33% and upon annealing in an oxygen flow at 400, 600, and 800 °C transformed to porous polycrystalline ZnO films. For oxygen contents ≥33% the obtained films were dense and polycrystalline with a 00.2 dominating orientation. X-ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), and Hall effect measurements were applied to characterize the obtained films.