The impact of oxygen deficiency on physical properties of Na2/3CoO2−δ has been investigated. From the combined thermogravimetric, magnetic susceptibility, and synchrotron x-ray Laue diffraction studies, it is demonstrated that Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98, which has the same Co valence as that of Na0.71CoO2, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of √12a. It is proposed that the Na ion ordering found in Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large √12a×√12a×3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling.