Combine 1 1/4 cups oats, 1 cup flour, baking powder, baking soda, salt, 1 tsp cinnamon, and nutmeg; stir well.
Combine sweet potato and the next 5 ingredients; stir well. Add the sweet potato mixture to the oat mixture, stirring just until dry ingredients are moistened. Stir in 1/3 cup of the chopped pecans.
Spoon the batter into greased and floured muffin pans, filling three-fourths full. Sprinkle topping evenly over the batter. Bake at 400 degrees for 15 minutes or until a wooden pick inserted in the center comes out clean. Remove from pans immediately.
Combine 1/4 cup oats, 1/4 cup flour, 1/4 cup brown sugar, 1/4 cup chopped pecans, and 1 tsp ground sugar in a small bowl. Stir well. Cut in the butter with a pastry blender until the mixture resembles coarse meal. Top the muffins with the mixture before baking.