Preheat the oven to 375°F. Lightly grease a 10-inch ovenproof skillet and set aside.
In a mixing bowl, combine rhubarb, granulated sugar, and cornstarch. Toss until the rhubarb is well coated, then spread evenly in the prepared skillet.
In the same bowl, mix oats, brown sugar, and flour. Cut in the butter using a pastry blender or your fingertips until the mixture is crumbly and resembles coarse crumbs.
Sprinkle the oat crumble topping evenly over the rhubarb filling.
Bake for 35–40 minutes, or until the topping is golden brown and the edges are bubbling.
Cool slightly before serving. Enjoy warm with vanilla ice cream or whipped cream.
Notes
As a note, a 9x9 square baking dish would also be fine.