The electrical properties of WO3 thin films vary significantly depending on the growth conditions. In this work, the influence of O-2 gas on the band gap of WO3 thin films during growth was investigated via electronic structure characterization using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). A substantial decrease in the electrical conductivity of the WO3 films was observed with an increase in the O-2 partial pressure during growth. Spectral differences in the peak energy and intensity were apparent for WO3 films grown under only Ar and those grown in Ar:O-2. It is difficult to explain the acquired spectrum of WO3 with oxygen defects through the rigid-band model in terms of the simple addition of electrons to the conduction band of WO3. Our results show that an oxygen deficiency in WO3 moves the conduction band to the Fermi edge.