Preheat the oven to 350°F (175°C). Lightly spray or butter a 7×11-inch or 8.5×11-inch baking dish.
In a bowl, mix the flour, oats, cinnamon, brown sugar, and melted butter.
Press half of this crumb mixture into the prepared baking dish.
Add chopped rhubarb on top of the pressed crumb mixture.
In a separate bowl, whisk together granulated sugar, cornstarch, and water. Microwave for 3-5 minutes until thickened and clear. Stir in vanilla extract.
Pour the sugar sauce over the rhubarb in the baking dish.
Evenly spread the remaining crumb mixture over the top.
Bake for 30-40 minutes until the edges are bubbling and the topping is golden brown.