Metal-organic frameworks (MOFs) have been identified as one of the promising electrocatalysts for the oxygen evolution reaction (OER). However, direct observation of the electrocatalytic behavior of MOF-based electrocatalysts remains extremely challenging, which is of great significance to understand their electrocatalytic mechanism. Herein, we developed a vertically oriented Ni-based MOF nanosheet array doped with 2.09 wt% Ce (denoted as Ce-NiBDC/OG). Ce-NiBDC/OG displayed a low overpotential of 265 mV to deliver a 10 mA cm-2 current density for the OER. In situ spectroscopy and operando microscopy visualized the phase transformation behavior of Ce-NiBDC/OG to Ce-doped NiOOH induced by electrochemical activation, which was regarded as the real active site. Mechanistic studies revealed that, for the Ce-NiBDC/OG-derived catalyst, the doping of Ce species in NiOOH significantly increased the adsorption of *OH, and further reduced the energy barriers of the rate-determining step (*OH→*O).