Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large bowl, toss the sliced rhubarb with granulated sugar, 3 tablespoons of flour, and vanilla extract until evenly coated.
Spread the rhubarb mixture evenly into the prepared baking dish.
In a separate medium bowl, combine the brown sugar, oats, 1 cup flour, and ground cinnamon.
Cut the cold butter into the dry mixture using a pastry cutter or fork until the mixture resembles coarse crumbs.
Sprinkle the oat topping evenly over the rhubarb filling.
Bake for 45 minutes, or until the topping is golden brown and the rhubarb filling is bubbling at the edges.
Remove from oven and allow to cool for at least 15 minutes to let the juices set before serving.