Prepare Ingredients: Measure and organize all components. Preheat oven to 350°F (175°C). Lightly grease an 8-inch square baking dish with butter or nonstick spray.
Make the Filling: In a large mixing bowl, combine diced rhubarb, granulated sugar, flour, water, cinnamon, and allspice (if using). Stir gently until rhubarb is evenly coated. Set aside.
Prepare the Topping: In a separate large bowl, whisk together rolled oats, brown sugar, and flour. Add chilled butter cubes. Using a pastry cutter or fork, blend until the mixture resembles coarse crumbs with pea-sized butter pieces.
Assemble the Crisp: Press half of the oat topping firmly into the bottom of the prepared baking dish to form a base. Spread the rhubarb filling evenly over the crust. Sprinkle the remaining oat topping over the rhubarb, covering it completely.
Bake: Place the dish on the center oven rack and bake for 40–45 minutes, or until the filling is bubbling vigorously at the edges and the topping is golden brown.
Cool & Serve: Remove from the oven and let rest for 15–20 minutes before serving. This allows the filling to thicken slightly. Serve warm, optionally accompanied by vanilla ice cream or whipped cream.