Sub‐10 nm CoFe₂O₄ nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one‐pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe₂O₄ towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe₂O₄ nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co_0.5Ni_0.5Fe₂O₄ exhibited a low overpotential of approximately 340 mV at 10 mA cm⁻², a smaller Tafel slope of 51 mV dec⁻¹, and stability over 30 h. The unique tunability of these CoFe₂O₄ nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis.