Preheat the oven to 350°F.
In a large mixing bowl, combine the chopped rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice. Stir until the rhubarb is well coated.
Pour the rhubarb mixture into a greased 9x13-inch baking dish.
In another bowl, mix the rolled oats, flour, ground cinnamon, brown sugar, and melted butter until crumbly.
Sprinkle the oat mixture evenly over the rhubarb layer in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the topping is golden brown and the rhubarb is bubbling.
Let it cool for about 10 minutes before serving.