1Preheat oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.
2Mix rhubarb, 1 tablespoon flour, sugar, water, cinnamon, and allspice together in a bowl. Mix oats, brown sugar, and 1/2 cup flour together in a separate bowl; add butter and mash into the oats mixture with a pastry cutter until crumbly.
3Press about half the oats mixture into the bottom of the prepared baking dish. Spread rhubarb mixture over the oats mixture; top with remaining oats mixture.
4Bake in preheated oven until rhubarb layer is set, about 45 minutes.