Combine oatmeal, flour, baking powder, and salt in a bowl.
Cream the butter and sugar in another bowl.
Add the egg and mix well.
Add the oatmeal mixture to the butter mixture and knead into dough.
Fold in chocolate chips or nuts.
Form the dough into evenly sized balls and place them on a baking tray.
Bake in a preheated oven at 180℃ for 12–15 minutes.
Cool on a wire rack.