The development of bio-derived composites represents a sustainable and cost-effective strategy for advanced energy storage applications. In this work, a porous carbon/nickel oxide (NiO) composite was synthesized from orange peel via carbonization at 500 °C followed by KOH activation at 700 °C and subsequent hydrothermal NiO modification. The resulting material exhibited a hierarchical porous structure with a high specific surface area (2120 m² g⁻¹ for OP_500_700 and 1968 m² g⁻¹ for NiO-modified OP_500_700_0.1M), with both values being significantly higher than that of the non-activated OP_500 (3.40–18.12 m² g⁻¹). Electrochemical evaluation revealed that the NiO-functionalized composite achieved a specific capacitance of 306.0 F g⁻¹ at 5 mV s⁻¹ and 281.5 F g⁻¹ at 2 A g⁻¹, surpassing the pristine activated carbon (281.9 F g⁻¹ and 259.6 F g⁻¹, respectively). In addition, both electrodes demonstrated excellent cycling stability, retaining more than 80% capacitance after 5000 charge–discharge cycles at a high current density of 20 A g⁻¹, while the NiO-modified electrode further benefited from a self-activation effect leading to >100% retention. These findings emphasize the synergistic effects of hierarchical porosity and NiO pseudocapacitance, establishing orange peel-derived carbon/NiO composites as scalable and sustainable electrode materials for next-generation supercapacitors.