Nanoparticles of Bi₃Ir, obtained from a microwave-assisted polyol process, activate molecular oxygen from air at room temperature and reversibly intercalate it as oxide ions. The closely related structures of Bi₃Ir and Bi₃IrO_x (x≤2) were investigated by X-ray diffraction, electron microscopy, and quantum-chemical modeling. In the topochemically formed metallic suboxide, the intermetallic building units are fully preserved. Time- and temperature-dependent monitoring of the oxygen uptake in an oxygen-filled chamber shows that the activation energy for oxide diffusion (84meV) is one order of magnitude smaller than that in any known material. Bi₃IrO_x is the first metallic oxide ion conductor and also the first that operates at room temperature.