Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish.
In a mixing bowl, combine the chopped rhubarb, 1/2 cup granulated sugar, 1 tablespoon flour, and vanilla extract. Stir until the rhubarb is well coated. Spread the mixture evenly in the prepared baking dish.
In another bowl, mix the rolled oats, 1 cup flour, brown sugar, cinnamon, and the remaining 1/2 cup granulated sugar. Pour in the melted butter and stir until the mixture is crumbly.
Sprinkle the oat mixture evenly over the rhubarb layer in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the topping is golden brown and the rhubarb is bubbling. Let it cool for at least 10 minutes before serving.