Oilseed cakes (OC), generated as by-products of mechanical oil extraction, are increasingly recognized as functional feed ingredients in swine production. Besides providing protein and residual lipids, they contain bioactive compounds such as polyphenols, tocopherols, and phytosterols that may exert antioxidant and immunomodulatory effects. This narrative review is based on a narrative literature search conducted in the Web of Science, Scopus, PubMed, and Google Scholar databases, covering publications from 2000 to 2026. This review summarizes current knowledge on the role of OC in pig nutrition, focusing on redox balance, immune modulation, and production performance. Experimental studies indicate that rapeseed, soybean, sunflower, and other oilseed cakes can influence oxidative stress markers and antioxidant enzyme activity, contributing to protection against oxidative damage. Evidence also suggests that these by-products may affect immune responses through improvements in intestinal barrier integrity and modulation of cytokine production. Among the reviewed OC, rapeseed, soybean, sunflower, and flaxseed cakes showed the most consistent evidence of beneficial effects on antioxidant status, immune function, and production performance, particularly when included at moderate dietary levels. In contrast, the available evidence for camelina, cottonseed, Amarula, and high-fiber oilseed by-products remains limited or inconsistent. Their effects appear to depend largely on dietary inclusion level, processing method, and the presence of antinutritional factors. Excessive dietary inclusion may impair nutrient digestibility and growth performance, highlighting the importance of optimizing inclusion levels according to the specific type of OC. Overall, OC represent a promising component of sustainable pig-feeding strategies, supporting animal health and production efficiency while promoting the use of agro-industrial by-products.