Preheat oven to 350°F (175°C).
In a bowl, mix flour, oats, granulated sugar, brown sugar, baking powder, and salt.
Add melted butter and mix until crumbly.
Press half of the mixture into a greased 9x9 inch baking pan.
In another bowl, combine berries, lemon juice, and cornstarch.
Spread berry mixture over the crust.
Sprinkle remaining crumb mixture on top.
Bake for 45 minutes or until golden brown.
Let cool before cutting into bars.