The (As-Ga)(2)-O-As defect in oxygen-doped GaAs, consisting of two As antisites neighboring an O center substitutional to As, is addressed through hybrid functional calculations. This defect not only accounts for the nearest neighbor environment of the O atom and the observed charge states but also yields a Fermi-level pinning position and optical transition energies between charge states in excellent agreement with experiment. The present assignment strongly supports the (As-Ga)(2)-O-As center as origin of the Fermi-level pinning in oxygen-doped GaAs. (C) 2013 AIP Publishing LLC.