Mix oats, milk, sugar, brown sugar, cinnamon, egg, and baking powder in a large mixing bowl or bowl of a stand mixer. Cover and place in the refrigerator overnight.
Combine topping ingredients, leaving the butter out for now, and place in a cool, dry place or plastic bag overnight for use in the morning.
When ready to bake, remove oatmeal ingredients from refrigerator and place in a well greased 9x13 baking dish. Gently fold in fresh or thawed blueberries.
Bake at 350℉ for 15 minutes.
While oats are baking, melt butter and add to topping ingredients. Stir in until well combined. Texture should be like a thick granola.
Remove oats from oven after baking 15 minutes and add topping, pressing gently into the oatmeal with a fork or spatula.
Bake an additional 20-25 minutes or until topping is golden brown.
Instant Bake Method
Combine all the oatmeal ingredients, except the blueberries, in a large mixing bowl or bowl of a stand mixer. Place in a well greased 9x13 baking dish and gently fold in blueberries.
Bake at 350℉ for 15 minutes.
Combine all the topping ingredients (same bowl you used for oatmeal can be used). Stir until well combined. The texture should be like a thick granola.
Remove oats from oven and top with streusel topping, pressing some of it down into the oat mixture.
Place back in oven and bake an additional 20-25 minutes or until topping is golden brown.