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High-entropy oxides (HEOs) exhibit excellent structural stability, and the synergistic effects among their constituent elements contribute to performance enhancement, providing vast opportunities for designing novel and efficient electrocatalysts. Utilizing four common transition metals such as Fe, Co, Ni, and Cr as the base and introducing a fifth metal, M (M = Mg, Mn, Zn, and Cu), with different outer electron shells, relevant high-entropy oxides with varying sizes were successfully synthesized via the hydrothermal method. Among all considered samples, FCNCMg, with the smallest particle size, possessed a much larger specific surface area, leading to the exposure of more active sites and thus favoring an improvement of the OER performance. The introduction of the fifth metal induced a redistribution of charges among Fe, Co, Ni, and Cr within the catalyst, resulting in different oxidation states for these metals. Mg addition led the XPS peaks of Fe/Ni and Cr/Co elements to shift towards lower and higher binding energies, respectively. This dynamic compensation effect between metal ions synergistically promoted their OER performance. In alkaline environments, FCNCMg exhibited the best OER performance, with an overpotential of 278 mV at 10 mA cm−2 and a Tafel slope of 49.90 mV dec−1. Moreover, it demonstrated excellent stability, achieving 50 h at 10 mA cm−2 and 20 h at 50 mA cm−2. This study, by controlling the composition of HEOs, explored the impact of particle size and synergistic effects on their performance, offering new insights for design and optimization of relevant high-performance HEOs. • A series of spinel (Fe 0.2 Co 0.2 Ni 0.2 Cr 0.2 M 0.2) 3 O 4 materials were successfully synthesized. • The introduction of M (M = Mg, Mn, Zn, and Cu) leads to tunable oxidation states of other elements. • The dynamic compensation effect among metal ions promots the OER activity of the catalysts. • FCNCMg exhibits an overpotential of 278 mV at 10 mA cm−2 and a Tafel slope of 49.90 mV dec−1. • FCNCMg can sustain stably for 50 h at 10 mA cm−2 and 20 h at 50 mA cm−2. [ABSTRACT FROM AUTHOR]