Fast Li-ion conducting garnets have shown excellent performance as chemically stable solid state Li electrolytes even at room temperature. However, because of phase formation and Li loss during preparation, reliably obtaining high Li-ion conductivities remains challenging. In this work, we show that an additional defect chemical species needs to be considered, namely, oxygen vacancies. We prove the existence of oxygen vacancies in all six investigated sample types: Ta-, Al-, and Ga-stabilized cubic Li7La3Zr2O12 (LLZO) polycrystals and Ta-stabilized LLZO single crystals. Isotope exchange three-dimensional analysis was used to characterize surface oxygen exchange (k*) and bulk oxygen diffusion (D*) enabled by the oxygen vacancies present in the LLZO variants. Remarkably high k* values of 10–11–10–8 cm s–1 and D* values of 10–15–10–11 cm2 s–1 were found at 350 °C in air. In a further data analysis, the differences between the compositions are investigated, the concentration of oxygen vacancies is estimated, and the possible effects on the cation defect chemistry and phase formation of LLZO are discussed.