We investigated the dependence of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to determine the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers positively correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+ / NS. The thermodynamic origins of these bilayer heights were investigated using molecular dynamics simulations. The overpotential (eta) for the OER correlates with the interlayer spacing; NS/Cs+ /NS has the lowest eta (0.45 V), while NS/Li+/NS exhibits the highest eta (0.68 V) for OER at a current density of 1 mA/cm(2). Kinetic parameters (eta and Tafel slope) associated with NS/Cs+ /NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.