Whisk together the pistachios, flour, oats, baking powder, and 1/4 teaspoon salt in a medium bowl; set aside. Put butter, brown sugar, and 1/4 cup granulated sugar in the bowl of an electric mixer fitted with the paddle attachment, mix on medium speed until creamy.
Stir pistachio mixture into butter mixture until combined. Work mixture through your fingers until it forms coarse crumbs ranging in size from small peas to gum balls; set topping aside.
Stir together cherries, remaining 1/2 cup granulated sugar, the cornstarch, cinnamon, and a pinch of salt in a medium bowl. Transfer cherry mixture to an 8-inch square baking dish. Sprinkle the topping evenly over cherry mixture. Bake until topping turns golden and juices are bubbling, 50 minutes to one hour. Let cool on a wire rack one hour before serving.