Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking dish.
In a large bowl, combine chopped rhubarb, sugar, cornstarch, and vanilla extract.
Pour the rhubarb mixture into the prepared baking dish.
In another bowl, mix oats, flour, brown sugar, cinnamon, and salt.
Stir in melted butter until crumbly.
Spread the oat mixture evenly over the rhubarb.
Bake in the preheated oven for 40 minutes or until golden brown.
Remove from the oven and let cool for 10 minutes.
Serve warm with vanilla ice cream on top.