Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish.
In a large bowl, combine the chopped rhubarb, granulated sugar, cornstarch, vanilla extract, and cinnamon. Mix well and pour the mixture into the prepared baking dish.
In another bowl, mix the rolled oats, flour, brown sugar, melted butter, and salt until crumbly.
Sprinkle the oat mixture evenly over the rhubarb filling in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the topping is golden brown and the rhubarb is bubbly.
Allow to cool for at least 10 minutes before serving.