Cream butter and sugar, then add the flour and oatmeal to make the dough (it will be a little crumbly). Set aside. Stir juice and zest into milk. Set aside. Butter (or spray) a 9 X 13 inch pan. Press two-thirds to three-quarters of the dough into the pan. Spread lemon/milk mixture evenly over the top of the dough. Sprinkle the remaining dough (kind of like streusel) over the top. Bake at 350 degrees for 30 to 35 minutes until golden. When cool, slice into 1-inch squares.