Although O3‐NaFe1/2Mn1/2O2 delivers a large capacity of over 150 mAh g−1 in an aprotic Na cell, its moist‐air stability and cycle stability are unsatisfactory for practical use. Slightly Na‐deficient O3‐Na5/6Fe1/2Mn1/2O2 (O3‐Na5/6FeMn) and O3‐Na5/6Fe1/3Mn1/2Me1/6O2 (Me = Mg or Cu, O3‐FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist‐air stability. O3‐Na5/6FeMn, O3‐FeMnCu, and O3‐FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g−1, respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3‐FeMnMg participate in the redox reaction more apparently than O3‐Na5/6FeMn. X‐ray diffraction studies unveil the formation of a P‐O intergrowth phase during charging up to >4.0 V.