Oxygen-17 magnetic resonance, in conjunction with high-temperature X-ray diffraction (XRD) and differential thermal analysis (DTA), were used to investigate the structure of Ba2In2O5 and the dynamics of oxygen ion motion between room temperature and 1200-degrees-C. NMR and thermal analysis demonstrate that at 925-degrees-C there is an order-disorder transition which involves oxygen atoms between layers of octahedrally coordinated indium atoms. Both NMR and X-ray diffraction show that the material retains an orthorhombic (layered) structure until approximately 1075-degrees-C, at which point the material becomes cubic. The number of mobile oxygen atoms in the structure increases continuously between 925 and 1075-degrees-C, and only above 1075-degrees-C does the full population of anions become mobile. These results imply that vacancies contribute to transport two-dimensionally within the tetrahedral layers at the order-disorder transition.