The atomic and electronic structure of nonstoichiometric oxygen-deficient tantalum oxide TaOx<2.5 grown by ion beam sputtering deposition was studied. The TaOx film content was analyzed by x-ray photoelectron spectroscopy and by quantum-chemistry simulation. TaOx is composed of Ta2O5, metallic tantalum clusters and tantalum suboxides. A method for evaluating the stoichiometry parameter of TaOx from the comparison of experimental and theoretical photoelectron valence band spectra is proposed. The charge transport properties of TaOx were experimentally studied and the transport mechanism was quantitatively analyzed with four theoretical dielectric conductivity models. It was found that the charge transport in almost stoichiometric and nonstoichiometric tantalum oxide can be consistently described by the phonon-assisted tunneling between traps.