Preheat oven to 400ºF and line a large rimmed baking sheet with foil. Make filling: Combine sugar, rhubarb and cornstarch in a large bowl. Let stand, stirring occasionally, until sugar and cornstarch are dissolved, about 10 minutes.
Make topping: Pulse oats, brown sugar, flour, cinnamon and butter in food processor until well combined (pieces should be no bigger than pea-size).
Spoon fruit into 6 6-oz. ramekins. Scatter topping evenly over filling. Place ramekins on lined baking sheet and bake until fruit is bubbling and topping is browned, 25 to 30 minutes. Transfer to wire rack to cool. Serve warm and topped with whipped cream, if desired.