Heat oven to 375°F. Lightly butter an 8-inch square baking dish.
Mix rhubarb, granulated sugar, cornstarch, flour, lemon juice, and vanilla in a large bowl until coated.
Spread the rhubarb mixture into the prepared dish and let it sit while you make the topping.
Stir oats, brown sugar, flour, cinnamon, and salt in a bowl. Drizzle in melted butter and toss until clumpy.
Scatter the oat mixture evenly over the rhubarb, covering to the edges.
Bake until the topping is deep golden and the fruit is bubbling thickly, 35 to 40 minutes.
Cool 15 minutes to set the juices. Serve warm, ideally with vanilla ice cream or lightly sweetened whipped cream.