Combine oatmeal, flour, and baking powder in a bowl.
In another bowl, cream together the butter and sugar.
Add the egg and mix well, then add the oatmeal mixture and knead into a dough.
Stir in additional ingredients such as chocolate chips or raisins and mix evenly.
Shape the dough into balls and place them on a baking sheet.
Bake in a preheated oven at 180℃ for 15 minutes.