Whisk brown sugar, white sugar, 1/4 cup oats, 1 teaspoon flour, and a pinch of cinnamon together in a bowl. Add rhubarb and blueberries; stir to coat. Pour mixture into a small baking dish.
Stir 1/2 cup flour, 1/3 cup oats, cinnamon, and nutmeg together in a bowl; add enough butter to create a crumbly topping, about 1/2 of the butter. Sprinkle crumbly topping over rhubarb mixture, and drizzle remaining butter over the top.
Bake in the preheated oven until golden and bubbling, 20 to 30 minutes. Cool for 5 minutes before serving.
