Intense interest in the Li-O2 battery system over the past 5 years has led to a much better understanding of the various chemical processes involved in the functioning of this battery system. However, detailed decomposition of the nanostructured Li2O2 product, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure operando under realistic electrochemical conditions. Here, we report operando nanobeam X-ray diffraction experiments that enable monitoring of the decomposition of individual Li2O2 grains in a working Li-O2 battery. Platelet-shaped crystallites with aspect ratios between 2.2 and 5.5 decompose preferentially via the more reactive (001) facets. The slow and concurrent decomposition of individual Li2O2 crystallites indicates that the Li2O2 decomposition rate limits the charge time of these Li-O2 batteries, highlighting the importance of using redox mediators in solution to charge Li-O2 batteries.