Designing efficient and stable electrocatalysts to improve the oxygen evolution reaction (OER) with slow reaction kinetics is essential to improve hydrogen production from electrochemical water splitting. This research has prepared a series of FeOOH/Ni(HCO₃)₂ heterostructured materials (denoted as FeOOH_x/Ni(HCO₃)₂) by a one-pot solvothermal method. The OER performance of the catalysts was maximized by finely tuning the content of different components, heterogeneous interfaces, and electronic structures. Specifically, the obtained FeOOH_0.60/Ni(HCO₃)₂ heterostructured nanosheets had the lowest overpotential of 216 mV at a current density of 10 mA·cm⁻² and were stable at a high current density of 100 mA·cm⁻² for more than 96 h. The excellent OER activity and stability were still maintained in alkaline natural seawater (1 M KOH + seawater). When FeOOH_0.60/Ni(HCO₃)₂ was used as the anode for water splitting, the electrolyzer provided a current density of 10 mA·cm⁻² at a very low cell voltage of 1.51 V (1.56 V at 1 M KOH + seawater) and exhibited superior stability. The outstanding OER performance is ascribed to the synergistic effect of FeOOH and Ni(HCO₃)₂ upon heterostructure formation, as well as the altered electronic structure between the heterogeneous interfaces and the suitable hierarchical nanosheet morphology facilitating many active sites. This work provides a promising direction for improving the electrocatalytic activity of nickel-based catalysts in seawater splitting, which has important implications for both hydrogen economy and environmental remediation.