Preheat your oven to 400 degrees Fahrenheit.
Grease a 12-cup standard-size muffin tin or line it with muffin liners.
In a large bowl, whisk together the all-purpose flour, rolled oats (reserve the extra 3 tablespoons for topping), brown sugar, baking powder, baking soda, salt, and cinnamon.
In a separate medium bowl, combine the leftover cranberry sauce, milk, vegetable oil, and egg.
Pour the wet ingredients into the dry ingredients and stir just until blended.
Divide the batter between the 12 muffin cups, filling them about three-quarters full.
Sprinkle the reserved rolled oats on top of each muffin.
Place the muffin tin in the oven and bake for 20 minutes.
Once done, remove the muffins from the oven and let them cool in the pan for about five minutes.
Carefully take the muffins out of the tin and serve warm.