The selection of oxide materials for catalyzing the oxygen evolution reaction in acid-based electrolyzers must be guided by the proper balance between activity, stability and conductivity-a challenging mission of great importance for delivering affordable and environmentally friendly hydrogen. Here we report that the highly conductive nanoporous architecture of an iridium oxide shell on a metallic iridium core, formed through the fast dealloying of osmium from an Ir₂₅Os₇₅ alloy, exhibits an exceptional balance between oxygen evolution activity and stability as quantified by the activity-stability factor. On the basis of this metric, the nanoporous Ir/IrO₂ morphology of dealloyed Ir₂₅Os₇₅ shows a factor of ~30 improvement in activity-stability factor relative to conventional iridium-based oxide materials, and an ~8 times improvement over dealloyed Ir₂₅Os₇₅ nanoparticles due to optimized stability and conductivity, respectively. We propose that the activity-stability factor is a key "metric" for determining the technological relevance of oxide-based anodic water electrolyzer catalysts.