Nonaqueous magnesium–oxygen (or “Mg-air”) batteries are attractive next generation energy storage devices due to their high theoretical energy densities, projected low cost, and potential for rechargeability. Prior experiments identified magnesium oxide, MgO, and magnesium peroxide, MgO2, as the primary discharge products in a Mg/O2 cell. Charge transport within these nominally insulating compounds is expected to limit battery performance; nevertheless, these transport mechanisms either are incompletely understood (in MgO2) or remain a matter of debate (in MgO). The present study characterizes the equilibrium conductivity associated with intrinsic (point) defects within both compounds using first-principles calculations. For MgO, negative Mg vacancies and hole polarons—the latter localized on oxygen anions—were identified as the dominant charge carriers. However, the large formation energies associated with these carriers suggest low equilibrium concentrations. A large asymmetry in the carrier mobility is predicted: hole polarons are highly mobile at room temperature, while Mg vacancies are essentially immobile. Accounting for nonequilibrium effects such as frozen-in defects, the calculated conductivity data for MgO is shown to be in remarkable agreement with the three “Arrhenius branches” observed in experiments, thus clarifying the long-debated transport mechanisms within these regimes. In the case of MgO2, electronic charge carriers alone—electron and hole polarons—are the most prevalent. Similar to MgO, the equilibrium concentration of carriers in MgO2 is low, and moderate-to-poor mobility further limits conductivity. If equilibrium behavior is realized, then we conclude that (i) sluggish charge transport in MgO or MgO2 will limit battery performance when these compounds cover the cathode support and (ii) what little conductivity exists in these phases is primarily electronic in nature (i.e., polaron hopping). Artificially increasing the carrier concentration via monovalent substitutions is suggested as a strategy for overcoming transport limitations.