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The addition of minor amounts of fluorides of alkaline and earth-alkaline elements to the oxides of magnesium, calcium, and aluminum causes oxygen ion conduction in the oxide-fluoride mixtures (OFM) at high temperatures. Selected OFM compositions were analyzed by X-ray diffraction, and scanning electron microscopy, and energy dispersive X-ray techniques, and the transport of oxygen ions in sintered OFM specimens was investigated by electrical current and electromotive force measurements of concentration cells with OFM electrolytes. It is concluded that the oxygen ion transport is accommodated by anion vacancies at oxide grain boundaries along twin interphase regions and in a percolating mode. Special requirements for establishing good electrical contacts to the grain boundary regions of OFM solid electrolytes for oxygen ion transport are discussed. OFM compositions, which contain alkali fluorides, may exhibit additional bulk oxygen ion conduction. The investigated OFM compositions do not show noticeable electronic conduction at low oxygen activities and their high thermodynamic stability makes them candidates for solid electrolytes in high-temperature electrochemical cells.