Gather all ingredients.

Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.

Mix together rhubarb, sugar, flour, water, cinnamon, and allspice in a large bowl until combined. Set aside.

Make topping: Mix together oats, brown sugar, and flour in another large bowl. Use a pastry cutter to incorporate butter into oat mixture until crumbly.

Press about 1/2 of the oat topping into the bottom of the prepared baking dish.

Top with rhubarb mixture; cover with remaining topping.

Bake in the preheated oven until filling is set and top is lightly browned, about 45 minutes. This homemade rhubarb crisp recipe, featuring an irresistible oat topping, is the perfect balance of sweet and tart. Perfect for rhubarb season, this easy treat is sure to be the dessert of the summer. Keep this recipe handy—you're going to want to make it again and again.