We investigate early-stage Nb oxidation by combining first-principles calculations with high-resolution transmission electron microscopy. An ordered ε phase is identified, in which O atoms occupy octahedral interstitial sites in an alternating layered configuration. Oxygen incorporation proceeds through geometrically open surface sites and is directed toward octahedral interstitial sites. A descriptor-based analysis shows that O–O separations within this interstitial sublattice capture the dominant trend in incorporation stability. As oxygen concentration increases, unfavorable short-range O–O interactions become unavoidable, leading to a rapid increase in free energy and providing a geometric mechanism that limits further oxygen incorporation.