In many organic reactions, the O2 activation process involves a key step where inert ground triplet O2 is excited to produce highly reactive singlet O2. It remains elusive what factor induces the change in the electron spin state of O2 molecules, although it has been discovered that the presence of noble metal nanoparticles can promote the generation of singlet O2. In this work, we first demonstrate that surface facet is a key parameter to modulate the O2 activation process on metal nanocrystals, by employing single-facet Pd nanocrystals as a model system. The experimental measurements clearly show that singlet O2 is preferentially formed on {100} facets. The simulations further elucidate that the chemisorption of O2 to the {100} facets can induce a spin–flip process in the O2 molecules, which is achieved via electron transfer from Pd surface to O2. With the capability of tuning O2 activation, we have been able to further implement the {100}-faceted nanocubes in glucose oxidation. It is anticipated that this study will open a door to designing noble metal nanocatalysts for O2 activation and organic oxidation. Another perspective of this work would be the controllability in tailoring the cancer treatment materials for high 1O2 production efficiency, based on the facet control of metal nanocrystals. In the cases of both organic oxidation and cancer treatment, it has been exclusively proven that the efficiency of producing singlet O2 holds the key to the performance of Pd nanocrystals in the applications.