Oat hull composites have been prepared by the free radical polymerization of regular or modified natural oils with divinylbenzene, n-butyl methacrylate, and maleic anhydride in the presence of dried, ground oat hulls. Parameters, such as cure time, filler load, cure temperature, and the fatty acid composition of the conjugated natural oil, have been investigated and related to the structure and properties of the composites. Structure–property relationships have been determined with the use of tensile tests, DMA, TGA, and Soxhlet extraction, followed by 1H NMR spectroscopic analysis of composite extracts. The best overall properties have been obtained for tung oil-based composites bearing 80 wt% of oat hulls cured at 160 °C for 4 hours.