Quick - Need Help Oatmeal Subsitute?

I am making cookies and just have 1 cup oatmeal, I need 1.5 Cups. What will happen, do I replace flour, or do nothing, I am looking for the cookie to come out flat and chewy and tender - (like those prepackaged Little Debbie oatmeal sandwich cookies with the white frosting) I live too far from town.