Preheat your oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease it generously.

In a large bowl, whisk together the flour, 1/2 cup rolled oats, granulated sugar, brown sugar, baking powder, baking soda, cinnamon, and salt. Make a well in the center of the dry ingredients.

In a separate medium bowl, whisk together the egg, buttermilk, vegetable oil, and vanilla extract. Pour the wet ingredients into the well in the dry ingredients. Stir just until combined; do not overmix. Gently fold in the diced rhubarb.

If making the streusel topping: In a small bowl, combine 1/4 cup rolled oats, 2 tablespoons flour, 2 tablespoons brown sugar, and 1/4 teaspoon cinnamon. Cut in the cold butter with a pastry blender or your fingertips until the mixture resembles coarse crumbs.

Divide the muffin batter evenly among the 12 prepared muffin cups. If using, sprinkle the streusel topping generously over each muffin.

Bake for 18-25 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean. Muffin tops should be golden brown. Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.