Boron-containing transition metal materials are new kinds of promising electrode materials for supercapacitors due to their good electrochemical activity, cost-effectiveness, and eco-friendliness. Here, through the oxidation of amorphous porous Ni-Co bimetallic borate nanosheets/C nanoaggregate precursor, a series of Ni-Co-B-O/C nanocomposites are fabricated. By adjusting the oxidation temperatures, the concentration of the oxidized-borate species near the surface was well-tuned. This results in creating different concentrations of the "electron-unsaturated" O atoms, which can accept extra electrons easily and promote the absorption/diffusion of electrolyte ions. Meanwhile, the hierarchically porous characteristic and low-crystallinity structure of the precursor are successfully maintained. Employed as an electrode, the optimized Ni-Co-B-O/C nanocomposite shows a specific capacity of 1139 C g(-1) at 1 A g(-1) and 83.5% capacitance retention after 2000 cycles. When assembled with reduced graphene oxide into a hybrid supercapacitor, a high energy density of 48.2 W h kg(-1) at a power density of 429.8 W kg(-1), high rate capability, and excellent stability were achieved, indicating great application potential in energy storage applications.