Understanding the structural nature of the active sites in electrocatalysis is central to discovering general design rules for better catalysts in fuel cells and electrolyzers. Nanostructures are widely used as electrocatalysts, but the location and structure of the active sites within the nanostructure are often unknown. This information is hidden in conventional bulk measurements due to ensemble averaging, hindering direct structure-activity correlation. Herein, we use a single-entity electrochemical approach to reveal the heterogeneity in electrocatalysts via scanning electrochemical cell microscopy (SECCM). Using hematite (α-Fe2O3) nanorods as the model catalyst for oxygen evolution reaction (OER), the electrocatalytic activity is measured at individual nanorods. Finer mapping within a single nanorod shows that the OER activity is consistently higher at the body portion vs the tip of the nanorod. Our results directly suggest the benefit of synthesizing longer hematite nanorods for better OER performance. The origin of the enhanced local activity is explained by the larger fraction of {001} facet exposed on the body compared to the tip. The finding goes beyond OER on hematite nanorods, highlighting the critical role of single-entity activity mapping and colocalized structural characterization in revealing active sites in electrocatalysis.