Preheat the oven to 350℉. Line a 9X9-inch pan with parchment paper, leaving parchment paper to hang over the sides of the pan.
To Make the Crust/Topping:
Combine the oats, flour, brown sugar, and baking powder in a medium-sized bowl. Pour in the melted butter and stir until crumbly but moist. Pour approximately ⅔ of the mixture into the prepared pan and press to form a crust. Set the remaining mixture aside for the topping later.
To Make the Filling:
In a medium-sized saucepan, stir together the raspberries, sugar, cornstarch, and lemon juice. Cook over medium heat, stirring often, for about 5 minutes or until thickened and no longer milky-looking. Immediately pour the filling over top of the crust and spread around. Crumble the reserved crust mixture over top.
Bake in the preheated oven for 35-40 minutes or until the filling is bubbly and the topping is golden. Let cool fully before serving as bars or serve it warm with ice cream (like a crisp).