Add 1 ½ cups flour, baking powder, salt, baking soda, and nutmeg. Stir just to combine. Add nuts if desired.
Separately, mix together the brown sugar, oats, 2 tablespoon flour, cinnamon, and oil until the mixture is evenly moist to make the streusel.
Grease the cups of a muffin pan or line with paper cups; divide batter evenly between the cups. They will be rather full. Top with the streusel.
Bake at 375F for about 20 minutes, until the tops are browned and a toothpick inserted into one comes out clean.