Sodium-ion batteries (Na-ion batteries) have emerged as a promising alternative to lithium-ion batteries due to sodium’s natural abundance, low cost, and compatibility with existing lithium-based infrastructure. As the global demand for cost-effective and sustainable energy storage solutions increases, Na-ion systems are gaining considerable attention. However, their commercial viability is currently hindered by challenges such as low energy density, poor rate performance, and limited long-term cycling stability. To address these limitations, the development of advanced electrode materials and functional additives is crucial. In this study, an oxygen-rich nanostructured graphene oxide (OR-NGO) material was synthesized via a modified Hummers method and incorporated into hard carbon (HC) anodes to improve electrochemical performance. Unlike typical carbon-based additives that rely primarily on increasing surface area, OR-NGO enhances the electrode function through its high concentration of redox-active oxygen functional groups and increased edge site exposure. These characteristics facilitate additional pseudocapacitive Na⁺ storage and improve both ionic and electronic transport at the electrode–electrolyte interface. To optimize its effect, three additive loadings (0.5, 1.0, and 1.5 wt%) were evaluated, with the 1.0 wt% OR-NGO sample exhibiting the most favorable electrochemical characteristics. The composite electrode demonstrated enhanced reversible capacity, superior rate capability, and improved cycling retention compared to pristine hard carbon. These findings highlight the synergistic role of chemical functionalization and nanoscale structuring in OR-NGO, underscoring its potential as a high-performance additive. This strategy offers a promising pathway for advancing sodium-ion battery technologies toward practical applications in large-scale energy storage systems.