Preheat the oven to 350°. Grease a nine inch pie pan with cooking spray.
In a bowl, combine the chopped rhubarb, brown sugar, flour, cinnamon, and orange juice. When they are fully mixed together, pour the mixture into the prepared pie plate.
Sprinkle one to two teaspoons of cornstarch on top of the filling. This creates a seal between the rhubarb and the crumble so that it doesn't get soggy.
In a seperate bowl, combine the brown sugar and the melted butter. Add the cinnamon, vanilla, flour and rolled oats. Stir until completely combined. Sprinkle the crumble on top of the rhubarb filling.
Bake the rhubarb crisp for 45-55 minutes or until the crumble is golden brown.
Serve with ice cream and enjoy!