Gather all ingredients. Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.
Combine rhubarb, white sugar, 2 tablespoons flour, egg, and ½ teaspoon cinnamon together in a large bowl until evenly coated.
Spread rhubarb mixture into the prepared baking dish.
Mix 1 ½ cups flour, brown sugar, oats, and ½ teaspoon cinnamon together in a separate bowl. Cut in butter using a pastry cutter or two forks until topping is crumbly. Sprinkle topping over rhubarb layer.
Bake in the preheated oven until hot and bubbling, about 40 minutes.
Serve hot or cold.
