Thermal treatments under a very wide range of oxygen pressures were used to probe the composition and defect nature of a lithium-overstoichiometric "Lix0CoO2" (x0 > 1) sample using X-ray powder diffraction, 7Li NMR, and electrochemical tests. It was found that at 900 C, under atmospheric and elevated oxygen pressures, the lithium-overstoichiometric sample gradually transformed to stoichiometric LiCoO2 by losing excess lithium in the form of Li2O. In addition, it was shown that the defect associated with Co2+ and oxygen deficiency as reported by Gorshkov et al. and Karelina et al. had a different NMR signature than that present in Li-overstoichiometric samples. Therefore, it is believed that oxygen vacancies are present but Co2+ ions are not present in Lix0CoO2 (x0 > 1). This leads to a formula [Li]interslab[CoIII1-3tCo3+(IS)2tLit]slab[O2-t], involving an intermediate spin configuration for 2t Co3+ ions in a square-based pyramidal site. This new model was supported by the NMR and magnetic data of the lithium-overstoichiometric sample and its deintercalated compounds. The effect of the defect on the end-of-discharge voltage profile during cycling was also discussed.