Tungsten oxide (WO3) thin films were deposited both on silicon, soda-lime glass and W foils by Ar/O2 plasma diode sputtering process from a W metal target. The influence of O2 content percentage on the structural and optical properties of the films, as well as the effects of post treatment annealing both in vacuum (400 °C) and in air (600 °C) have been investigated. X-ray diffraction studies revealed that the as-grown films are amorphous-like regardless of the oxygen percentage. The degree of crystallinity of films is increased by a post-growth thermal-annealing procedure. With respect to other plasma sputtering recipes, here a lower stress state is favoured by the slower deposition rate and the multi-step deposition. The optical band gap deduced from the absorbance spectra ranges from 3.1–3.3 eV for the amorphous samples and it decreases to 2.3–2.5 for the more crystalline films. The photoelectrochemical activity of WO3 samples annealed at 600 °C in air have been investigated as a function of the O2 content.