We report a colloidal synthesis strategy for producing Mn_xO_y@Ni(OH)₂ nanoheterostructures under mild conditions, i.e., low temperature and ambient pressure. The role of carboxylic acid ligands in directing the synthesis is systematically explored, revealing that lower ligand concentrations along with low-molecular weight molecules favor the formation of well-defined Mn_xO_y@Ni(OH)₂ heterostructures. Electrochemical characterization demonstrates that the resulting nanocomposites exhibit significantly enhanced electrochemical surface area and oxygen evolution reaction (OER) activity compared to their single-component counterparts. Specifically, Mn_xO_y@Ni(OH)₂ achieves a low overpotential of 299 mV at 10 mA cm^–2, a Tafel slope of 61 mV dec^–1, and a low charge transfer resistance of 9 Ω. The improved OER performance is attributed to the synergistic effect between the Ni(OH)₂ nanosheets, which facilitate *OOH intermediate formation, and the MnO₂ component, known for its intrinsic catalytic activity. Additionally, Mn₃O₄ serves as a stabilizing phase and precursor to MnO₂, contributing to the overall durability and structural integrity of the catalyst.