The naturally occurring oxide of GaAs has a high density of defects that pin the Fermi level at the GaAs surface. The principle electronic defect causing the Fermi level pinning is widely believed to be an arsenic antisite double donor. We have used scanning tunneling microscopy to show that the arsenic antisite defects are formed during the initial period of oxidation of GaAs(001) by atomic oxygen. Atomic oxygen displaces a single arsenic atom in the top layer leaving behind an undimerized arsenic atom with a half-filled dangling bond. The displaced arsenic atoms bond to remaining arsenic dimers to form arsenic antisites. Cluster calculations confirm the assignment of the oxygen binding sites and the role of undimerized single arsenic atoms as acceptor sites.