Preheat oven to 350°F. Grease a 9 x 9-inch baking dish and set it aside.
Mix the filling: In a large bowl, combine the rhubarb, white granulated sugar, instant tapioca, and orange juice. Stir to combine and pour into the prepared baking dish.
5 cups chopped rhubarb, ¾ cup granulated sugar, 2 tbsp + 1 tsp instant tapioca, ⅓ cup fresh orange juice
Make the streusel: In the same large bowl, combine the oats, flour, brown sugar, and cinnamon. Stir to combine and add the butter. Using your hands or a pastry cutter, rub the butter pieces into the mixture until it resembles coarse cornmeal with pieces of butter the size of a pea or smaller. Pour the streusel mixture over the rhubarb and spread it to an even layer.
1 cup old-fashioned oats, ¾ cup all-purpose flour, ½ cup light brown sugar, 1 ½ tsp ground cinnamon, 8 tbsp (1 stick) unsalted butter
Bake in the preheated 350°F oven until bubbling throughout, 40-55 minutes. Remove from the oven and cool 10-15 minutes before serving.