It is of great significance to develop novel superhydrophilic/underwater superoleophobic materials using a facile and simple method for efficiently separating oily wastewater. In this work, a completely inorganic hierarchical structure made up of a Co and Ni double hydroxide (CoNi–OH) was successfully prepared using a simple one-step in situ method. Combining the unique hierarchical architecture with strong water locking capability, a biomimetic cactus-like hierarchical CoNi–OH-4 coated stainless steel mesh (CoNi–OH-4/SSM) exhibits excellent superhydrophilicity/underwater superoleophobicity and superior anti-oil-fouling capability for high-efficiency oil/water separation with an ultrahigh flux (>37 227 L m⁻² h⁻¹) and excellent oil rejection ratio of >99.95%. Continuous lubricating oil/water separation tests demonstrated that CoNi–OH-4/SSM shows high separation capability and excellent recycling over 20 cycles with a stable flux of 37 000 L m⁻² h⁻¹ and oil rejection ratio of >99.90%. Furthermore, CoNi–OH-4/SSM exhibits outstanding mechanical, chemical, and long-term stability. Biomimetic CoNi–OH-4/SSM, made using a simple preparation method, has superior anti-oil-fouling properties, excellent oil/water separation performance, and outstanding mechanical and chemical stabilities, revealing its great potential for practical applications in oil/water separation.