Layered sodium transition metal oxides are attractive cathodes for sodium-ion batteries, yet their practical implementation is severely hindered by rapid air and moisture induced degradation. While previous stabilization approaches have relied on surface coatings or indirect lattice modification, strategies that stabilize the oxygen framework through controlled cation incorporation in the Na layer remain largely unexplored. Here, we introduce an oxygen-affinitive bismuth substitution strategy to stabilize O3-type NaNi0.3Fe0.4Mn0.3O2 cathodes through oxygen-affinitive Bi incorporation in the Na layer, which stabilizes the oxygen framework. Detailed structural and surface analyses demonstrate that Bi incorporation suppresses Na dissolution, water insertion, and oxygen-vacancy formation. Consequently, the Bi-substituted cathode preserves its layered structure, surface integrity, and electrochemical reversibility after humid air exposure, whereas the pristine material becomes electrochemically inactive. Importantly, electrochemical and kinetic analyses confirm that this enhanced environmental stability is achieved without compromising intrinsic Na+ transport. This work establishes oxygen-framework engineering via oxygen-affinitive cation incorporation as a previously underexplored and practically relevant design principle for air-tolerant, high-performance layered sodium-ion battery cathodes.