The pre-oxidized crystalline (3 x 1)-O structure of InAs(100) has been recently found to significantly improve insulator/InAs junctions for devices, but the atomic structure and formation of this useful oxide layer are not well understood. We report high-resolution photoelectron spectroscopy analysis of (3 x 1)-O on InAs(100) and InSb(100). The findings reveal that the atomic structure of (3 x 1)-O consists of In atoms with unexpected negative (between -0.64 and -0.47 eV) and only moderate positive (In2O type) core-level shifts; highly oxidized group-V sites; and four different oxygen sites. These fingerprint shifts are compared to those of previously studied oxides of III-V to elucidate oxidation processes. (C) 2015 AIP Publishing LLC.