A composite plugging agent, glucose-derived carbon spheres grafted with poly (methyl methacrylate-co-octadecyl methacrylate-co-styrene) (GCS-g-PMOS), was synthesized by grafting an oil-absorbing polymer onto glucose-derived carbon spheres through suspension polymerization for application in oil-based drilling fluids. This material was designed to overcome the limited adaptability of conventional rigid plugging agents by integrating the structural support of carbon spheres with the swelling and filling capabilities of the polymer phase. Response surface methodology was used to optimize the preparation parameters, and the optimum formulation was determined to be 0.75 wt% glucose-derived carbon spheres, 1.5 wt% N,N′-methylenebisacrylamide, and 2.0 wt% benzoyl peroxide. Under these conditions, the oil absorption capacity reached 9.02 g/g. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and wettability analyses confirmed successful grafting of the polymer phase and showed that the composite possessed good thermal stability and strong oil affinity. After aging at 160 °C, the addition of 6 wt% GCS-g-PMOS reduced the American Petroleum Institute (API) filtrate loss from 4.4 to 2.4 mL and decreased the filter cake thickness from 2.0 to 0.9 mm. The material also exhibited effective plugging performance in media of different scales and reduced the permeability of the filter cake, with a maximum permeability reduction rate of 63.53%. These results demonstrate that the integration of rigid carbon spheres and an oil-absorbing polymer is beneficial for forming a dense sealing structure and enhancing fluid-loss control in oil-based drilling fluids under high temperature conditions.