Preheat the oven to 350℉. Line a 9X9-inch pan with parchment paper, leaving parchment paper to hang over the sides of the pan.
Whisk together the oats, flour, brown sugar, and baking powder in a large bowl. Cut in the cold butter cubes with a pastry cutter until the butter pieces are smaller than peas. Press ½ of this mixture into the prepared pan and set aside the rest for later.
In a medium-sized saucepan, stir together the sugar, cornstarch, cold water, and lemon zest. Add the blueberries and cook over medium heat, stirring often, for about 5 minutes. The mixture should be thickened and no longer milky. Pour the hot filling over top of the crust and crumble the reserved crust mixture over top.
Bake in the preheated oven for 28-30 minutes or until golden brown and fragrant. Serve warm with ice cream or cool with whipped cream. Enjoy!