The electrical properties of hafnium dioxide (HfO2) thin films are often attributed to the oxygen composition and oxygen-related defects; however, there have been few systematic studies on the electronic structures of such oxygen atoms. In this study, we used electron energy-loss spectroscopy to identify the influence of the electronic states of the oxygen atoms in HfO2 thin films by comparing HfO2 samples for different oxygen source pulse time during atomic-layer deposition (ALD). Although all samples by ALD have higher oxygen content in the film than that in the reference stoichiometric HfO2 sample, variations in the local symmetry of amorphous HfO2 thin films were significantly affected by oxygen source pulse time. Moreover, leakage currents of high-oxygen content HfO2 samples with longer O3 pulse time decreased considerably, compared with those of low-oxygen content HfO2 samples with shorter O3 pulse time, in which oxygen-related defects were observed by the Vfb shift under constant voltage stress. After postdeposition annealing (PDA), the electronic structure of oxygen atoms in HfO2 films was affected by the initial oxidation states in the amorphous HfO2 films. Furthermore, after PDA, polycrystalline HfO2 in high-oxygen content samples was mostly of the monoclinic phase, whereas the metastable tetragonal phase was readily formed in low-oxygen content HfO2.