Preheat oven to 350 degrees. In a blender, process 1 1/2 cups oats into a fine meal. Whisk together ground oats, cornstarch, cinnamon, baking powder, and salt.
In a large bowl, using a mixer, beat butter and sugars until light and fluffy. Beat in eggs and vanilla. With mixer on low, gradually add oat mixture, beating until combined. Fold in chocolate chips, raisins, and remaining 3 cups oats.
In batches, drop dough in 2-tablespoonful mounds onto a parchment-lined baking sheet, 2 inches apart. Bake until cookies are golden brown at edges, 15 to 18 minutes. Let cool on sheet on a wire rack 2 minutes, then transfer to rack and let cool completely.
Chock-full of chocolate and raisins, these oatmeal cookies won't disappoint. And by using oat flour in place of wheat flour, everyone can get in on the fun.