In a large bowl, combine the oatmeal, flour, baking soda and salt. Set aside.
In a glass bowl, combine the sweetened condensed milk, chocolate, butter and 1/2 tsp. of the salt. Microwave until melted. Stir to blend.
In a very large bowl, beat the softened butter until fluffy. Beat in the brown sugar til blended.
Beat in the eggs and vanilla.
Gradually beat in the dry ingredients, on low speed, until blended.
Grease a 10 x 15 inch pan with softened butter or coconut oil. Pat two-thirds of the mixture into the bottom of the pan. You don't need to go up the sides. Just cover the bottom of the pan. It can help to moisten your fingers with a little water when you're patting in the crust.
Spoon the chocolate mixture onto the bottom crust and spread so it covers the crust.
Cover as much as possible with blobs of the remaining dough.
Bake 25-30 minutes. Be careful not to overbake.
Remove from the oven. Let cool completely before cutting.