Preheat oven to 375 degrees F.
Line a 12 cup muffin pan with papers.
In a medium bowl, stir all topping ingredients together. Set aside.
In a large mixing bowl, stir together the flour, brown sugar, baking powder, salt and cinnamon.
In a separate, smaller bowl, stir together the milk, oil and vanilla. Pour the milk mixture into the bowl with the flour mixture and stir just until blended. Do not overmix. Gently fold in the raspberries, and rhubarb.
Divide up the mixture into the muffin cups. The cups should be very full, but not overflowing. Sprinkle the tops with the oat topping.
Bake for 22-24 minutes, or until the muffins are lightly browned on the tops and a toothpick inserted into the center of a muffin comes out clean.
Place muffins on a cooling rack and allow to cool completely before removing from tins.