Stable and metastable oxide structures resulting from the reaction of GaAs(001)-(2x4) with O atoms are investigated using scanning tunneling microscopy (STM). The relative stabilities of these oxide structures are examined using density functional theory calculations (DFT). STM images show that when GaAs(001)-(2x4) is exposed to O atoms, the O atom will either displace an arsenic atom from its original dimer position by taking its place or chemisorb subsurface and create a metastable site. As the O atom coverage increases, O atoms increasingly occupy the position of two displaced arsenic atoms across two neighboring dimers, while the number of subsurface sites remains constant. These experiments show that oxygen preferentially removes an arsenic pair (As-2), with one atom originating from each of two neighboring As-As dimers instead of from the same As-As dimer. DFT calculations demonstrate the relative stability of the final chemisorption state by calculating the most stable oxygen configurations as functions of the oxygen and arsenic chemical potentials. (C) 2003 American Institute of Physics.