In a large bowl, stir together the oats, sugar, baking powder, and salt.
In a smaller bowl, stir together the milk, egg, melted and cooled butter, and vanilla extract.
Combine the wet ingredient with the dry in the large bowl, stir to combine. Then stir in the blueberries.
Add this mixture to a pie pan, cover and store overnight in the refrigerator.
In a small bowl, cut together the crumb topping ingredients. The mixture will be crumbly, like small peas you can press together. Store overnight (or you can make this in the morning, either way).
Top the oatmeal with the crumb topping, then bake at 350°F for 30-35 minutes. If the crumb topping doesn't look golden brown and slightly crispy, you can set under the broiler for a minute or two.