The cleanup of oil-contaminated water remains an environmental challenge, particularly in applications where affordability, biodegradability, and reusability are crucial. In this study, we present a range of novel, bio-derived hydrogel composites from agricultural cellulose reinforced with functional additives for efficient oil–water separation. Four composites were synthesized and tested: raw cellulose hydrogel (S1), hydrogel reinforced with coated mesh support (S2), hydrogel with activated carbon reinforcement (S3), and hydrogel blended with plant powder (S4). Water absorption tests were conducted in oil–water systems at five contact times (5 to 120 minutes) to evaluate separation efficiency. The best performance was exhibited by the activated carbon hydrogel (S3), with a maximum absorption of 174.6% at 5 minutes and a high average of 128.12%, owing to the synergistic effect of the porous carbon skeleton and the hydrophilic cellulose matrix. The plant powder hydrogel (S4) also performed well with an average absorption of 83.25% and has great potential as a natural reinforcement. The coated mesh hydrogel (S2) displayed good absorption (66.17%) and mechanical stability, while raw cellulose hydrogel (S1) presented poor long-term efficiency due to the collapse of its structure. These results illustrate the potential of low-cost, biodegradable hydrogel composites for oil–water separation. The synergy between natural and functional additives was successful in dramatically enhancing water selectivity and retention, enabling the development of scalable and green separation materials for environmental remediation.