High-resolution photoemission is applied to the oxygen adsorption on the Si(001) surface at 120 K and the subsequent evolution of the adsorbates upon annealing. Si 2p components due to the Si2+ and Si3+ species are observed from the very early stage of adsorption at similar to 120 K, which grow linearly with the oxygen coverage. This indicates an active agglomeration of oxygen adsorbates even for the submonolayer adsorption at low temperature. Annealing above 500 K enhances the agglomeration by mostly converting the Si1+ species into Si3+ and then into Si4+. In addition, the annealing changes the Si 2p binding energies for the Si2+ and Si3+ species by 0.14 and 0.23 eV, respectively. These shifts are attributed to the structural relaxation (strain relief) of the metastable oxygen-adsorbate complex formed at low temperature.