Preheat your oven to 375°F (190°C).
In a large bowl, mix together rhubarb, granulated sugar, and flour for the filling.
Pour the rhubarb mixture into a greased 9x13 inch baking dish.
In a separate bowl, combine oats, brown sugar, flour, and melted butter for the topping.
Sprinkle the oat mixture evenly over the rhubarb filling.
Bake for 35 to 40 minutes, until the topping is golden brown and the filling is bubbly.
Serve warm, optionally with ice cream or whipped cream.