Toss the rhubarb with 1 1/2 cups of sugar and the tapioca in a large bowl. Drizzle with the melted butter and toss again to coat. Pour the rhubarb into the pie crust, and set aside.
Mix together the oats, 1/4 cup sugar, flour, softened butter, and cinnamon in a small bowl until evenly blended. Sprinkle the oat topping over the rhubarb.
Bake in the preheated oven for 15 minutes, then reduce the heat to 325 degrees F (165 degrees C), and continue baking until rhubarb is tender enough to pierce with a fork, about 30 minutes more. Allow to cool to room temperature before serving.
