Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking pan.
In a large bowl, combine sugar, tapioca, cornstarch and salt. Place rhubarb and raspberries into bowl making sure to completely coat them with dry ingredients. Pour into baking pan.
In a medium bowl, mix together brown sugar, flour and oats. Cut in butter until mixture resembles pea-sized crumbs. Spread on top of fruit mixture.
Bake in preheated oven for 45 minutes, or until rhubarb is tender.