Photocatalytic selective oxidation reactions hold great promise for the design of high-value-added organic intermediates, but many of these reactions suffer from low conversion efficiency and selectivity due to uncontrollable oxidation processes. In view of using photogenerated reactive oxygen species as the key oxidant in a selective oxidation reaction, we propose that a highly selective oxidation reaction can be achieved by modulating the corresponding photocatalytic molecular oxygen (O2) activation processes. Using cubic indium sulfide (β-In2S3) nanosheets as a model system, we show that the charge carriers involved in O2 activation can be optimized with the introduction of surface S vacancies. Benefiting from the enhanced charge separation and transfer processes, the In2S3 nanosheets with S vacancies could simultaneously activate O2 into superoxide radicals via electron transfer under visible-light irradiation to display outstanding activity for the selective oxidation of alcohols to aldehydes with high conversion and selectivity. This study offers a new strategy to optimize photocatalytic selective oxidation reactions.