This study uncovers an overlooked oxidation mechanism featuring the *in situ formation of surface OH species on low-valence copper (Cu0/Cu+) catalysts utilizing air and water as green oxidants and media. By combining kinetic isotope effect (KIE) studies, in situ Raman spectroscopy, and DFT calculations, the research demonstrates how localized adsorbed hydroxyl radicals enable highly selective oxidation of biomass-derived aldehydes (such as glucose-to-gluconic acid conversion with up to 91% selectivity). These insights pave the way for designing efficient, nonprecious metal catalytic systems for sustainable biorefinery and fine chemical synthesis. Read the full paper in PNAS.