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 FeOOHx/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 FeOOH0.60/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 FeOOH0.60/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.