Mix flour, oats, corn starch and powdered sugar in a large bowl. Blend in butter. Work with hands until a soft, smooth dough forms. Shape into a disk and wrap with plastic wrap. Place in the refrigerator 30 minutes or until easy to handle. Preheat oven to 300F. Roll out dough to 1/4″ thickness. Cut into shapes with cookie cutters. Place on ungreased baking sheets. Decorate if desired. Bake for 15-25 minutes, or until edges are lightly browned. Time will depend on cookie size. Remove and cool completely on wire rack. Store in airtight container at room temperature.