In a blender or food processor, process 1½ cups oats into a fine meal.
Whisk together ground oats, cornstarch, cinnamon, baking powder, and salt.
In a large mixing bowl with a hand mixer or in the bowl of a stand mixer, beat butter and sugars together until light and fluffy. Beat in eggs and vanilla.
With mixer on low speed, gradually add oat mixture, beating until combined. Fold in chocolate chips, raisins, and remaining 3 cups oats.
Spray a baking sheet with non-stick cooking spray. Drop dough in 2 tablespoon mounds, 2 inches apart. Bake until cookies are golden brown at edges, approximately 15 minutes.
Let cookies rest on cookie sheet for about 2 minutes, then transfer cookies to wire rack to cool completely.
Store cookies in an airtight container at room temperature up to 5 days or freeze.