Unraveling the precise location and nature of active sites is of paramount significance for the understanding of the catalytic mechanism and the rational design of efficient electrocatalysts. Here, we use well-defined crystalline cobalt oxyhydroxides CoOOH nanorods and nanosheets as model catalysts to investigate the geometric catalytic active sites. The morphology-dependent analysis reveals a ~50 times higher specific activity of CoOOH nanorods than that of CoOOH nanosheets. Furthermore, we disclose a linear correlation of catalytic activities with their lateral surface areas, suggesting that the active sites are exclusively located at lateral facets rather than basal facets. Theoretical calculations show that the coordinatively unsaturated cobalt sites of lateral facets upshift the O 2p-band center closer to the Fermi level, thereby enhancing the covalency of Co-O bonds to yield the reactivity. This work elucidates the geometrical catalytic active sites and enlightens the design strategy of surface engineering for efficient OER catalysts.