In　bimetallic　heterostructured　nanoparticles　(NPs),　the　synergistic　effect　between　their　different　metallic　components　leads　to　higher　catalytic　activity　compared　to　the　activity　of　the　individual　components.　However,　how　the　dynamic　changes　through　which　these　NPs　adopt　catalytically　active　structures　during　a　reaction　and　how　the　restructuring　affects　their　activity　are　largely　unknown.　Here,　using　operando　transmission　electron　microscopy,　structural　changes　are　studied　in　bimetallic　Ni-Rh　NPs,　comprising　of　a　Ni　core　whose　surface　is　decorated　with　smaller　Rh　NPs,　during　a　CO　oxidation　reaction.　The　direct　atomic-scale　imaging　reveals　that,　under　O-2-rich　conditions,　Ni　core　partially　transforms　into　NiO,　forming　a　(Ni+NiO)-Rh　hollow　nanocatalyst　with　high　catalytic　activity.　Under　O-2-poor　conditions,　Rh　NPs　alloy　with　the　surface　of　the　core　to　form　a　NiRh-alloy　surface,　and　the　NPs　display　significantly　lower　activity.　The　theoretical　calculations　indicate　that　NiO　component　that　forms　only　under　O-2-rich　conditions　enhances　the　activity　by　preventing　the　CO　poisoning　of　the　nanocatalysts.　The　results　demonstrate　that　visualizing　the　structural　changes　during　reactions　is　indispensable　in　identifying　the　origin　of　catalytic　activity.　These　insights　into　the　dynamic　restructuring　of　NP　catalysts　under　a　reactive　environment　are　critical　for　the　rational　design　of　high-performance　nanocatalysts.