Oil palm empty fruit bunch (OPEFB) is a cellulose-rich lignocellulosic residue generated in large quantities by the palm oil industry, particularly in Malaysia and Indonesia, yet remains substantially underutilized. Converting OPEFB into cellulose nanocrystals (CNC) provides an opportunity to valorize this waste into high-value materials. This systematic review synthesized the literature on OPEFB-derived CNC following the PRISMA 2020 framework. Structured searches covering 2015–2026 combined OPEFB, cellulose nanocrystal/nanocrystalline cellulose, and extraction-technology terms. Thirty-nine eligible primary and secondary sources were synthesized through narrative analysis, comparative tables, and a PRISMA flow diagram. Sulfuric acid hydrolysis remains the most extensively studied extraction route, generally producing rod-like CNC with nanometre-scale diameters and crystallinity indices of approximately 55–70%. Emerging deep eutectic solvent (DES) approaches, particularly choline chloride-based systems combined with process intensification or solvent recovery, demonstrate promising reductions in production cost and global warming potential. TEMPO oxidation, enzymatic and ionic-liquid pretreatments, and ultrasonic- or microwave-assisted processes provide alternative advantages in selectivity, mild processing, or energy efficiency. However, trade-offs in yield, processing time, and complexity remain. OPEFB-derived nanocellulose has been explored for cementitious composites, polymer nanocomposites, adsorption, drug delivery, and flexible energy-storage applications. Key gaps include inconsistent reporting of yield and crystallinity, limited OPEFB-specific techno-economic and life-cycle assessments, and scarce pilot-scale validation. Future research should emphasize standardized reporting, solvent-recycling-integrated green extraction, continuous biorefinery processing, and integrated techno-economic and life-cycle assessments to support the development of a circular bioeconomy in palm-oil-producing regions.