We have optimized the synthesis of the well-known metal-organic framework Cu I -MFU-4l and studied its O2 binding properties under various conditions relevant to O2 capture from air in the presence of water vapor. Spectroscopic, magnetic, and computational analyses revealed that the copper(I) sites bind O2 via electron transfer to form copper(II)-superoxo species. Interestingly, the superoxo moieties bind in both side-and end-on modes at the copper(II) sites, and these modes are in equilibrium over a range of temperatures. Breakthrough cycling experiments indicate the material is stable to extended cycling under dry and humid air streams and reversibly captures O2 from ambient air in the presence of water. While both O2 and N2 rapidly adsorb in the material under these conditions, the activation barrier for O2 desorption is higher than that for N2 desorption, and this feature can be exploited to access high-purity O2 (>99%) after initial N2 desorption. Breakthrough analyses further indicate that the O2 capacity of the material is unaffected by the presence of humidity, suggesting co-adsorbed water does not bind to the exposed Cu I sites. These results highlight the advantages of using soft copper(I) sites in MOFs for selective O2 capture in the presence of water. Further, while high adsorption selectivity for O2 over N2 has traditionally been sought in candidate MOFs for O2-selective air separations, our results reveal that differences in the desorption kinetics can be used to access high purity O2 even when adsorption behavior suggests relatively low selectivity for O2. This discovery suggests that it may be worthwhile to reinvestigate existing materials that have been overlooked based on preliminary analysis of their O2 and N2 adsorption behavior. Research is ongoing in our laboratory to further advance Cu I -MFU-4l for practical air separations, including the scaleup of the materials developed here and the study of hydrophobic polymer 49 coatings to minimize water uptake.