As a four-electron transfer reaction, oxygen evolution reaction (OER) is limited by large overpotential and slow kinetics. Here, we in-situ synthesized two-dimensional (2D) Ni-Fe metal-organic framework nanosheets on nickel foam (Ni_xFe-TPA/NF, TPA = terephthalic acid) for oxygen evolution in alkaline and alkaline seawater electrolytes. In 1 M KOH, Ni₃Fe-TPA/NF shows a low overpotential (η₁₀) of 189 mV at 10 mA·cm⁻² and an ultra-low overpotential of only 260 mV at 500 mA·cm⁻². In alkaline seawater, Ni₃Fe-TPA/NF still provides impressive OER performance, with an η₁₀ of 265 mV. In-situ Raman characterization results show that the phase transition occurs during the OER, and Ni₃FeOOH with more oxygen vacancies is in-situ formed, reducing the OER energy barrier. Density functional theory (DFT) reveals that the synergy between Ni and Fe reduces the energy barrier and accelerates the rate-determining step. In addition, the ultra-thin 2D sheet structure and the close combination of Ni₃FeOOH and highly conductive NF support ensure the high catalytic OER activity. Therefore, the surface reconstruction and structural modification strategy can be used to design and prepare high-performance OER electrocatalysts for energy-related applications.