In-situ or operando quantitative analysis is very important for Li-O2 batteries, in order to properly, accurately and comprehensively evaluate electrocatalysts and characterize Li-O2 electrochemistry in real-time. Synchrotron XRD can provide much higher X-ray intensity and time resolution than traditional in-house diffractometers, and therefore can contribute to quantitative analysis for Li-O2 batteries. Here, operando synchrotron XRD is further developed to quantitatively study Li-O2 batteries with nano-catalysts, Ru/MnO2. The time-resolved oxygen evolution reaction (OER) kinetics for Li-O2 cells with Ru/MNT was systematically investigated using operando synchrotron radiation powder X-ray diffraction (SR-PXD). Li2O2 decomposition in the electrodes with Ru/MNT catalysts during galvanostatic and potentiostatic charge processes followed pseudo-zero-order kinetics and showed ideal Coulombic efficiency (close to 100%). Furthermore, it was found that the OER kinetics for a cell with 2 wt% Ru/MNT charged at a constant potential of 4.3 V was even faster than that for a cell with the same amount of pure Ru nanoparticles, which have been considered as a highly active catalyst for Li-O2 batteries. These results indicated that Ru/MNT with a special nanostructure represented a very efficient electrocatalyst for promoting the OER in Li-O2 batteries. We also demonstrate that synchrotron radiation XRD can “highlight” a way to quantitative analysis for Li-O2 batteries.