Optical and transport properties of nonstoichiometric tantalum oxide thin films grown by ion beam deposition were investigated in order to understand the dominant charge transport mechanisms and reveal the nature of traps. The TaOx films composition was analyzed by X-ray photoelectron spectroscopy and by quantum-chemistry simulation. From the optical absorption and photoluminescence measurements and density functional theory simulations, it was concluded that the 2.75 eV blue luminescence excited in a TaOx by 4.45 eV photons, originates from oxygen vacancies. These vacancies are also responsible for TaOx conductivity. The thermal trap energy of 0.85 eV determined from the transport experiments coincides with the half of the Stokes shift of the blue luminescence band. It is argued that the dominant charge transport mechanism in TaOx films is phonon-assisted tunneling between the traps.