Oxygen nonstoichiometry and electrical conductivity were measured on the system of gamma-NaxCoO2, allegedly so far the best p-type thermoelectric oxide, against oxygen activity (a(O2)) across its widest ever range below a(O2) = 1 for a fixed Na-content x = 0.706 at different temperatures in the range of 773-973 K, and at a fixed temperature 973 K for x = 0.664, 0.706 and 0.731, respectively. It has been deduced therefrom that as a(O2), decreases, the majority disorder type shifts from [V'(Na)] approximate to p to either [V'(Na)] approximate to 2[Co-Na(center dot center dot)] or [V'(Na)] approximate to 3[Co-i(center dot center dot center dot)], however, exhibiting a positive deviation from the ideal defect behavior. The latter is attributed to the positive deviation of holes due to their degeneracy. By taking into appropriate account of the activity coefficient of holes in terms of the Fermi-Dirac integral of order 1/2, the nonstoichiometry and electrical conductivity have been precisely and consistently depicted to evaluate the defect-chemical parameters including the effective mass and mobility of holes and the redox equilibrium constant. The phase-stability limit of gamma-NaxCoO2 is documented against temperature and Na-content.