Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.

In a large bowl, gently combine the chopped rhubarb, granulated sugar, flour, and vanilla extract until the rhubarb is evenly coated. Pour this mixture into the prepared baking dish, spreading it into an even layer.

Sprinkle the dry cake mix evenly over the top of the rhubarb filling. Do not stir or mix into the rhubarb layer.

If using, sprinkle the rolled oats evenly over the layer of dry cake mix.

Slowly drizzle the melted butter evenly over the entire surface of the cake mix, ensuring most of the dry mix gets some butter coverage. This helps it brown evenly.

Bake for 50 to 60 minutes, or until the topping is golden brown and the rhubarb filling is bubbling up around the edges. If the topping browns too quickly, loosely cover with aluminum foil for the last 15 minutes.

Remove from the oven and let cool for at least 30 minutes before serving. This allows the filling to set slightly. Serve warm, optionally with vanilla ice cream or whipped cream.