We studied oxidation at a dangling bond (DB) on the H-terminated Si(100) surface by the first-principles calculations. We found that oxidation easily occurs at the exposed DB on the H-terminated Si(100) surface. The dissociated O atoms are chemisorbed at a dimer bond and a back bond, resulting in adjacent H atom migration onto the DB. As a consequence of the alternate oxidation and subsequent H atom migration processes, the atomic wire oxidation is actually found to occur on the H-terminated Si(100) surface at low temperatures without desorbing H atoms, as observed in our scanning tunneling microscopy experiment.