Preheat oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.
Combine rhubarb, white sugar, 2 tablespoons flour, egg, and 1/2 teaspoon cinnamon together in a large bowl until evenly coated. Spread rhubarb mixture into the prepared baking dish.
Mix 1 1/2 cup flour, brown sugar, oats, and 1/2 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.
