Oat Okara Flour and Its Use for Nutritional Enhancement and Glycemic Reduction in Cookies

Oat okara, a protein and fiber‐rich by‐product of oat milk production, remains underutilized due to its high moisture content and spoilage risk. This study characterized the nutritional composition, functional properties, and phytochemical profile of dried oat okara flour (OOF) and evaluated its application in cookies at 10%–30 % wheat flour substitution. OOF contained 50.51 % carbohydrates, of which 40.80% is dietary fiber, 28.44% protein, 14.68% fat, 5.04% ash, and 36.11 ppm avenanthramides. It exhibited low solubility (27%) but strong water (2.00 g/g) and oil absorption capacity (1.13 g/g). Cookies enriched with OOF improved dietary fiber (up to 7.25%) and protein content (up to 14.24%) while maintaining similar energy values (528.66–538.92 kcal/100 g). In vitro starch digestibility decreased with higher OOF levels, lowering the predicted glycemic index by approximately 14%. At the highest substitution level, antioxidant stability was maintained through Day 14, phytochemical retention through Day 21, and texture through Day 28. These findings highlight the potential of OOF as a functional ingredient to improve nutritional value and reduce glycemic response in baked products. This work supports the valorization of plant‐based by‐products within circular food systems.