A comprehensive investigation of oxide ion transport in La1-x Sr (x) MnO3-0.5x (x = 0.2 and 0.5) over the temperature range of 800-2000 K is carried out employing long molecular dynamics simulations. The macroscopic structural and transport properties are in good agreement with previous experimental studies. The role of La/Sr ordering on oxide ion diffusivity is investigated. Fresh insights on diffusion pathways, microscopic energetics and mechanism of oxide ion transport are derived.