e investigate stau-antistau annihilation into heavy quarks in the phenomenological Minimal Supersymmetric Standard Model within the DMNLO project. We present the calculation of the corresponding cross section including corrections up to O(alphs_a) and QED Sommerfeld enhancement. The numerical impact of these corrections is discussed for the cross section and the dark matter relic density, where we focus on top-quark final states and consider either neutralino or gravitino dark matter. Similarly to previous work, we find that the presented corrections should be included when calculating the relic density or extracting parameters from cosmological observations. Considering scheme and scale variations, we estimate the theoretical uncertainty that affects the prediction of the annihilation cross section and thus the prediction of the relic density.