Sodium–oxygen batteries have drawn considerable attention due to their high specific energy and the high abundance of sodium. However, stable sodium–oxygen batteries currently require complex cathode formulations. Here, the first demonstration of a highly stable sodium–oxygen system comprising a simple carbon cathode, an ultradry electrolyte, and a newly designed separator that is both capable of blocking dendrites and is impenetrable to oxygen is reported. The battery shows remarkable rechargeability of more than 400 cycles at the current density of 0.28 mA m−2 to the capacity of 1.0 mAh cm−2. An oxygen diffusion study using a dye-assisted visual chemical reaction proves that the separator efficiently restricts oxygen crossover toward the anode. An in situ atomic force microscope study shows that the discharge product (NaO2) is stable against the electrolyte under anhydrous conditions for over 48 h in either argon or oxygen atmospheres. The sodium–oxygen battery also demonstrates small overpotential (<40 mV) upon charging and remarkable oxygen efficiency (>96%).