Oxygen-17 magnetic resonance, inconjunction with high-temperature X-ray diffraction (XRD) and differential thermal analysis (DTA), were used toinvestigate the structure of Ba2In205 and the dynamics of oxygen ion motion between room temperature and1200 C. NMR and thermal analysis demonstrate that at 925 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~ 1075 C, at which point the material becomes cubic. The number of mobile oxygen atoms in the structure increases continuously between 925 and 1075 C, and only above 1075 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.