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Gallium oxide (β-Ga 2 O 3 ) is becoming a popular material for high power electronic devices due to its wide bandgap and ease of processing. In this work, β-Ga 2 O 3 substrates received various annealing treatments before atomic layer deposition of HfO 2 and subsequent fabrication of metal–oxide–semiconductor (MOS) capacitors. Annealing of β-Ga 2 O 3 with forming gas or nitrogen produced degraded capacitance–voltage (C–V) behavior compared to a β-Ga 2 O 3 control sample with no annealing. A sample annealed with pure oxygen had improved C–V characteristics relative to the control sample, with a higher maximum capacitance and smaller flat-band voltage shift, indicating that oxygen annealing improved the C–V behavior. X-ray photoelectron spectroscopy also suggested a reduction in the oxygen vacancy concentration after O 2 annealing at 450 °C, which supports the improved C–V characteristics and indicates that O 2 annealing of β-Ga 2 O 3 may lead to better MOS device performance.