Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Prepare the filling: In a large bowl, gently toss the chopped rhubarb with granulated sugar, flour, vanilla extract, and cinnamon until evenly coated. Pour the mixture evenly into the prepared baking dish.

Prepare the topping: In a separate medium bowl, whisk together the flour, rolled oats, brown sugar, and salt for the topping.

Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Do not overmix.

Sprinkle the oat topping evenly over the rhubarb filling in the baking dish. Ensure the filling is mostly covered.

Bake for 45 to 55 minutes, or until the topping is golden brown and the rhubarb is bubbling thickly around the edges. If the topping browns too quickly, loosely cover with foil.

Remove from the oven and let the crisp cool for at least 20 minutes before serving. Serve warm, preferably with vanilla ice cream or whipped cream.