Step 1: Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish (or an equivalent 2-quart baking dish) with butter or cooking spray.
Step 2: In a large bowl, combine the chopped rhubarb with 3/4 cup granulated sugar, 1/4 cup cornstarch, and 1 teaspoon vanilla extract. Toss gently until the rhubarb is evenly coated.
Step 3: Pour the rhubarb mixture into the prepared baking dish and spread it evenly.
Step 4: In a separate medium-sized bowl, combine 1 cup all-purpose flour, 1/2 cup old-fashioned rolled oats, 1/2 cup packed light brown sugar, 1/2 teaspoon ground cinnamon, and a pinch of salt. Whisk these dry ingredients together thoroughly.
Step 5: Add 1/2 cup (1 stick) unsalted butter, cut into small cubes and chilled. Using your fingertips, a pastry blender, or a fork, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 6: Sprinkle the oat crumble topping evenly over the rhubarb filling in the baking dish. Make sure to cover the rhubarb completely.
Step 7: Place the baking dish on a baking sheet and bake for 40-45 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when pierced with a knife.
Step 8: Remove the crumble from the oven and let it cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set up. Serve warm.