Preheat oven 350°.
Line a 13 x 9 baking pan with non-stick foil.
In a mixer on low, combine flour, oats, 3/4 cup sugar and butter; mix until crumbly.
Set aside 1 1/2 cups crumbs for topping, and press the rest into the prepared pan.
Bake 15 minutes.
Cool completely.