Preheat oven to 375°F. Prepare a 10-inch skillet by coating it with butter or using non-stick spray.
Rhubarb Mixture
Place rhubarb in a bowl. Add brown sugar, flour, and cinnamon. Stir gently and make sure every piece of rhubarb is covered in the sugar mixture. Set aside.
Crisp Mixture
Combine the oats, flour, brown sugar, and cinnamon together. Add in the cubed butter and use your hands to combine. You want the butter to be pea-sized and coated entirely. You should have 4 cups.
Spread 2 cups over the bottom of your prepared skillet and press flat. Pour the rhubarb mixture over top and press down with rubber spatula. Pour remaining crisp mixture over top of the rhubarb and bake uncovered at 375°F for 35-45 minutes. If the crisp starts to brown too quickly, place foil over top and continue baking.
Notes
If you want to make this a Strawberry Rhubarb Crisp use 3 cups chopped rhubarb and 2 cups of cleaned, hulled, and quartered strawberries.