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Abstract Designing highly active and robust catalysts for the oxygen evolution reaction is key to improving the overall efficiency of the water splitting reaction. It has been previously demonstrated that evaporation induced self‐assembly (EISA) can be used to synthesize highly porous and high surface area cerate‐based fluorite nanocatalysts, and that substitution of Ce with 50% rare earth (RE) cations significantly improves electrocatalyst activity. Herein, the defect structure of the best performing nanocatalyst in the series are further explored, Nd₂Ce₂O₇, with a combination of neutron diffraction and neutron pair distribution function analysis. It is found that Nd^{3 +}cation substitution for Ce in the CeO₂fluorite lattice introduces higher levels of oxygen Frenkel defects and induces a partially reduced RE_1.5Ce_1.5O_{5 +x}phase with oxygen vacancy ordering. Significantly, it is demonstrated that the concentration of oxygen Frenkel defects and improved electrocatalytic activity can be further enhanced by increasing the compositional complexity (number of RE cations involved) in the substitution. The resulting novel compositionally‐complex fluorite– (La_0.2Pr_0.2Nd_0.2Tb_0.2Dy_0.2)₂Ce₂O₇is shown to display a low OER overpotential of 210 mV at a current density of 10 mAcm⁻²in 1M KOH, and excellent cycling stability. It is suggested that increasing the compositional complexity of fluorite nanocatalysts expands the ability to tailor catalyst design.