Molecular dynamics simulations of the effect of A-site cation disorder on oxygen diffusion in (Ba0.5Sr0.5)CoO2.5, (Ba0.5Sr0.5)FeO2.5 and (Ba0.5Sr0.5)Co0.8Fe0.2O2.5 were conducted to understand the oxygen diffusion mechanism. The diffusion coefficients of oxygen were strongly dependent upon the degree of A-site Ba/Sr cation ordering. The oxygen diffusion coefficient decreased and the oxygen diffusion activation energy increased with Ba/Sr cation ordering in the alternating (001) layers of the perovskite structure. The ordering of Ba/Sr cations also caused oxygen/vacancy ordering. In particular, vacancy location in the oxygen layers parallel to the Ba-rich layers significantly increased oxygen diffusivity in BSCF-related materials.