The oxidation of GaSb(100) surface has been widely studied because it affects the functionality of various devices. However even initial stages of the oxygen incorporation are not completely understood. To clarify this issue, we have investigated the oxidized GaSb(100) surfaces, which have been recently probed by scanning tunneling microscopy and spectroscopy, with high resolution synchrotron radiation photoelectron spectroscopy, in order to interconnect these different measurements. The results give a clear support that the oxidation initiates through saturation of available Ga bonds with; i.e., replacing some of the Sb-Ga bonds with O-Ga in the surface layers. Oxygen atoms have two different bonding environments in consistent with two dominating STM features. Also role of the plasmon features in the spectra have been elucidated. (C) 2016 Elsevier B.V. All rights reserved.