Abstract The monoclinic β-polymorph of gallium oxide is a semiconductor with an ultra-wide bandgap. It is becoming increasingly significant for various technological applications. We have investigated the tracer self-diffusion of oxygen in β-Ga₂O₃ single crystals as a function of the oxygen partial pressure (2, 20 and 200 mbar) at a temperature of 1375 °C. Isotopically enriched ¹⁸O₂ gas was used as a tracer source and secondary ion mass spectrometry to analyze depth profiles. We observed that, with decreasing oxygen partial pressure, the diffusivities at a given temperature increase significantly. We suggest that this behaviour can be explained by a change in the diffusion mechanism from oxygen interstitials to oxygen vacancies.