HighlightsDensity functional theory calculations indicates that interstitial B atoms can tune the atomic spacing of host metal Os and achieve a reversal hydrogen adsorption-distance relationship.The structure–activity relationship between the spacing of active Os atoms and catalytic activity is established.Prepared OsB2with increasing dOs-Osof 2.96 Å presents the optimal hydrogen evolution reaction activity (8 mV @ 10 mA cm−2) and robust stability in alkaline media.Supplementary InformationThe online version contains supplementary material available at 10.1007/s40820-023-01142-1.