Although chitosan (CS)- and essential oil (EO)-based packaging systems have been widely reviewed, a focused synthesis connecting nano-delivery design with interfacial regulation, film-network evolution, release behavior, and preservation performance in real food systems remains lacking. This review addresses that gap by examining CS-based nano-delivery systems for EOs in active food packaging, with an emphasis on how carrier design and multiscale organization govern functional performance. Major delivery strategies, including nanoemulsions, nanoparticles, nanogels, Pickering emulsions, nanofibrous systems, and nanocomposites, are discussed in relation to EO stabilization, dispersion uniformity, and controlled release. Their effects on film microstructure, mechanical and barrier properties, thermal stability, optical behavior, and antimicrobial and antioxidant activities are further evaluated alongside preservation outcomes in fruits, vegetables, dairy products, meat, and aquatic products. Particular attention is given to structure-function relationships across the carrier, interface, and film-network levels, and to the distinction between established active-packaging functions and emerging smart-packaging applications. Current challenges include EO compositional variability, limited cross-study comparability, sensory constraints, migration and regulatory concerns, and insufficiently scalable fabrication routes. Future work should prioritize mechanism-informed interfacial design, standardized evaluation frameworks, food-specific release-preservation correlations, and scalable green manufacturing.