In a large mixing bowl, cream together sugars and butters until fluffy. Add eggs and vanilla; mix until combined.
Stir in flour, 1 1/2 c. oats, baking soda and salt. Work in remaining 1 c. oats with hands or a spoon.
Reserve 1 1/2 c. oats mixture for topping. Press the remaining mixture into a greased 13×9 dish. Pour fudgy filling on top. Crumble reserved oat mixture and sprinkle on top.
Bake at 350 degrees for 30-35 minutes, until top is lightly browned. Cool and then refrigerate to firm completely.