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As one of the most common and important defects, O vacancy has a great influence on β-Ga2O3, which has not been studied systematically with appropriate functional approximation. Herein, the effects of O vacancies on the structural, electronic, and optical properties of bulk β-Ga2O3 were systemically investigated by using the shell DFT-1/2 method. For intrinsic β-Ga2O3, the calculated bandgap is 4.77 eV, which corresponds well to the experimental value. Band structures and density of states (DOS) prove that O vacancy will introduce a deeper donor level in the bandgap. Charge density distribution demonstrates the covalent properties of the Ga–O bond and reveals the charge transfer near the O vacancy. Phonon dispersion spectra prove the dynamical instability of the O vacancy systems. The formation energy curves show that the +2 charge state is energetically favorable at low Fermi energy, while the neutral defect is the most stable one at high Fermi energy. The transition energy of ɛ(+2/0) also proves that the O vacancy acts as a deep donor. The introduction of O defects introduces extra peaks in the lower energy regions of the absorption and electron energy loss spectra, and the peak positions are in good agreement with the previous experimental results. This study gives a point of view on the effects of O vacancies on β-Ga2O3 and proves the advantages of shell DFT-1/2 calculation in ultrawide bandgap semiconductors. [ABSTRACT FROM AUTHOR]