Preheat the oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, combine rolled oats, all-purpose flour, light brown sugar, baking powder, and salt; stir well.

Pour melted butter into the dry mixture and mix until crumbly, resembling wet sand.

Press about two-thirds of the oat mixture into the bottom of the prepared baking pan.

Toss mixed berries with granulated sugar, cornstarch, and vanilla extract in a separate bowl until coated.

Spread the berry mixture evenly over the crust and sprinkle the remaining oat mixture over the top.

Bake for 30 minutes or until the top is golden brown and the berry filling is bubbling.

Allow bars to cool completely in the pan before lifting out and slicing into squares.