In a large bowl or mixer, cream sugar and butter.
Add sourdough starter and oatmeal and mix together.
Add remaining dry ingredients, mix well, and chill in refrigerator several hours or overnight.
After chilling, preheat oven to 375°.
Drop dough in large, shaped spoonfuls onto a baking sheet and smash slightly.
Bake 12-15 minutes, removing to a wire rack to cool.