Developing high-efficiency and low-cost oxygen evolution reaction (OER) catalysts is crucial to advance the water splitting technology for sustainable hydrogen production. Here, a FeCoNi coordinated benzene-1,3,5-tricarboxylic acid (FeCoNiBTC) metal-organic framework (MOF) was synthesized by one-step solvothermal method for OER. A rapid in-situ chemical and electrochemical transformation was observed on the surface of the FeCoNiBTC MOF during OER process. The formed catalytic active FeCoNiO_x(OH)_y species retained the unique structure feature of initial FeCoNiBTC, moreover, it possessed multiple transition metal active nodes that cooperate with each other to adjust the electronic structure. Owing to the above structure advantages, the in-situ transformed FeCoNiO_x(OH)_y showed excellent OER catalytic activity with a small overpotential of 230 mV to achieve the 100 mA·cm⁻², a low Tafel slope of 50.2 mV·dec⁻¹, and superior stability of almost 80 h in alkaline aqueous solution. This work systematically studies the structure–performance relations of the multi-metal MOF-based materials in OER process, and it would enrich the exploration of highly efficient OER electrocatalysts.