The energy levels of the As-As dimer defect at GaAs/oxide interfaces are accurately determined within a hybrid functional scheme and aligned with respect to the GaAs band gap. An As-As dimer is constructed at a model interface between GaAs and kappa-Al2O3, which satisfies electron counting rules. The defect is studied through its charge transition levels which account for structural relaxation upon charging and which can directly be compared to measured defect energies. The antibonding state of the As-As dimer is found to lie at 0.30 eV below the conduction-band edge, in good correspondence with experimentally observed defect states. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816661]