The presence of metastable molecular precursor to dissociative oxygen chemisorption has been identified, for the first time, on Si(100) and Si(111) surfaces using ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy at 40 K. Density of valence states for such species shows three distinct peaks at 1.8, 3.4, and 4.9 eV below the Fermi energy of substrates. The precursor stabilized on Si(100) at 40 K induces a large work-function increase of 1.5 eV at saturation coverage. It also gives rise to a shoulder to O 1 s photoemission spectrum that is located at 1.2 eV lower binding energy than the O 1 s peak of stable and dissociated oxygen species. It has been concluded that the major reason of a larger work-function increment at 40 K is the existence of a molecular precursor, of peroxy radical type, on Si(100) and Si(III). These results strongly suggest that there exists a general mechanism of oxygen adsorption onto silicon surfaces.