To understand reaction mechanism of the oxidation at an Si(111)-7×7 surface using O₂ at 300 K, the determination of initial products is essentially important. Oxygen bonding configurations and silicon oxidation states were observed using real-time X-ray photoelectron spectroscopy for O 1s together with Si 2p. It is found that ins structure initially forms, where one oxygen atom inserts in the backbond of Si adatom. It is ascertained that the chemisorbed molecular oxygen, so-called paul oxygen, is the adsorbate on top of the ins structure. It is also clarified that ad-ins structure and ins-tri structure, where ad means an oxygen atom on top of Si atom and tri means the interstitial oxygen atom, appear after a short time. Our results imply the presence of a mobile O₂ on the surface.