Preheat the oven to 350°F (175°C). Lightly grease an 8-inch square baking dish with nonstick cooking spray.
In a large bowl, combine the rolled oats, baking powder, cinnamon, and salt.
In another bowl, whisk together the mashed bananas, milk, honey, egg, and vanilla extract until smooth.
Pour the wet ingredients into the oat mixture and stir until fully combined.
Gently fold in the blueberries without crushing them.
Transfer the mixture to the prepared baking dish and spread it into an even layer.
Bake for 30–40 minutes, or until the center is set and the top is lightly golden. Baking time is typically around 35–36 minutes.
Remove from the oven and allow the baked oatmeal to cool for 10–15 minutes.
Slice into squares and serve warm, or cool completely before refrigerating for meal prep