The initial stage of oxidation at an unpaired dangling bond (DB) site on a hydrogen-terminated Si(100)2 x 1 surface was studied using an ultrahigh-vacuum scanning tunneling microscope. The surface image was observed in situ during exposure to O-2 molecules at room temperature. During exposure the unpaired DBs induced structural changes around themselves, and they were mostly localized at one side of a Si dimer bond. We conclude that the backbonds near an unpaired DB were selectively oxidized, following the dissociation of O-2 molecules at the unpaired DB site. Based on high-resolution measurements, a model of a backbond-oxidized-structure was discussed.