Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or cooking spray.

In a large mixing bowl, combine all-purpose flour, rolled oats, brown sugar, baking soda, and salt. Mix well. Drizzle in melted unsalted butter until crumbly.

Press half of the oat mixture into the bottom of the greased baking dish.

In another bowl, whisk together granulated sugar and cornstarch. Add the chopped rhubarb and vanilla extract, stirring to coat.

Spread the rhubarb mixture over the crust. Sprinkle remaining oat mixture on top.

Bake for 35-40 minutes or until the top is golden brown and the filling is bubbly.

Let cool completely in the baking dish on a wire rack before slicing.