Preheat oven to 350°F (175°C).

In a large mixing bowl, combine rolled oats, all-purpose flour, brown sugar, granulated sugar, melted butter, salt, and baking powder.

Press two-thirds of the oat mixture into the bottom of a greased baking pan to form the crust.

In a separate bowl, toss fresh blueberries with granulated sugar, lemon juice, lemon zest, and cornstarch.

Spread the blueberry mixture evenly over the crust.

Crumble the remaining oat mixture on top of the blueberries.

Bake in the preheated oven for 30 minutes or until the topping is golden brown.

Allow to cool before cutting into bars.