The aim of the study was to investigate the properties of oat cookies fortified with poppy seed cake. The modification of the recipe and the supplementation with the by-product improved the nutritional value and maintained the overall sensory acceptability of the analyzed material. An increase in the amount of the poppy seed cake formulation (up to 15%) was accompanied by significant increases in the contents of individual components: protein (from 12.43 to 14.09%), fat (from 6.34 to 6.98%), and crude fiber (from 2.32 to 3.62%) compared to the control sample. In turn, the moisture level (from 34.8 to 32.86%) and the content of soluble sugars (from 18.7 to 16.15%) decreased. The addition of the poppy seed cake changed the texture properties of the product. Oat cakes supplemented with poppy seed cake were characterized by higher hardness and chewiness values and lower cohesiveness. The aroma and palatability of the product achieved greater acceptability with increasing amounts of poppy seed cake, whereas an opposite tendency was observed for changes in the appearance and texture of the modified oat cookies. The highest sensory rates were achieved by the sample containing 10% poppy seed cake. Cookies produced according to this recipe are recommended for industrial production (their texture and sensory properties are appropriate, and they contain polyphenols, flavonoids, and significant amounts of protein and crude fiber).