Atomic transport of oxygen in nonstoichiometric oxides is an extremely important topic that overlaps science and technology. In many cases, diffusion of oxygen controls sintering, grain growth, and creep. High oxygen diffusivity is critical for efficient operation of many fuel cells. Additionally, oxygen diffusivities are an essential ingredient in any point defect model. Secondary ion mass spectrometry (SIMS) is the most accurate modern technique for measuring oxygen tracer diffusion. This paper will briefly review the principles and applications of SIMS for the measurement of oxygen transport. Case studies will be taken from recent work on ZnO and selected high-temperature superconductors.