Rational design and understanding of the intrinsic mechanism are critical to develop highly active and durable electrocatalysts. In this study, a series of bi-metallic boride catalysts based on Ni and Co were prepared, and their activities were evaluated. The synthesised Co–10Ni–B catalyst exhibited excellent activity for water splitting in a 1 M KOH electrolyte. The overpotential was 330 mV at a current density of 10 mA cm⁻², better than previously reported mono-metallic borides and even IrO₂. The synergistic effect of Co and Ni was proved by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The facile formation of critical intermediates CoOOH and NiOOH during the catalytic processes and a significant increase in surface area owing to the introduction of a second metal into mono-metallic boride were attributed to the superior catalytic performance of catalysts for the oxygen evolution reaction. A Co–10Ni–B-sp catalyst with a higher surface area than the Co–10Ni–B catalyst was also synthesised to evaluate the effect of a high surface area on the catalytic activity. A lower overpotential of 310 mV at a current density of 10 mA cm⁻² was achieved.