Highly active, earth-abundant, and stable electrocatalysts are indispensable for overall water splitting. In this work, hierarchical dendrite-like CO0.75Ni0.2 alloy prepared by a simple hydrothermal reduction route can serve as a high-performance catalyst for hydrogen evolution reaction. The synergistic effect from Co and Ni atoms optimizes the catalytic behavior of their alloy, and the hierarchical dendrite structure enlarges the contact area with the electrolyte, exposing more active sites. Based on the dendrite-like CO0.75Ni0.2 alloy, we further fabricate a core-shell architecture with the conductive N-doped carbon shell coated dendrite-like CoNi/CoNiO2 nanocomposites via a calcination process at 600 degrees C (CoNi/CoNiO2@NC-600). The dendrite-like CoNi/CoNiO2@ NC-600 displays significantly improved activity of oxygen evolution reaction due to large active surface area, high conductivity, and the continuous electron transfer among metal, metal oxide, and N-doped carbon. Furthermore, the dendrite-like CO0.75Ni0.2 alloy and CoNi/CoNiO2@NC-600 are assembled into a water splitting electrolyzer with a low voltage of 1.51 V to drive a current density of 10 mA cm(-2).