Preheat your oven to 375°F.
Grease a 9×9-inch baking dish with butter or cooking spray.
Combine the chopped rhubarb, granulated sugar, 1/4 cup flour, vanilla extract, and cinnamon in a large bowl. Toss gently to coat the rhubarb evenly without breaking the pieces. Transfer the rhubarb mixture to the prepared baking dish, spreading it into an even layer.
In the same bowl, mix the oats, 3/4 cup flour, brown sugar, and salt for the topping. Add the cold cubed butter to the oat mixture. Use your fingers or a pastry cutter to work the butter into the dry ingredients until coarse crumbs form. Keep the butter cold for a flakier, crisper topping and don’t overmix.
Sprinkle the topping evenly over the rhubarb filling.
Bake for 35–40 minutes, or until the filling is bubbly and the topping is golden brown.
Remove the crisp from the oven and let it cool on a wire rack for 15 minutes before serving.