Preheat oven to 350 degrees. Combine flour, oats, brown sugar and butter in large bowl; mix well. This mixture will be crumbly.
Line a 13 x 9-inch baking pan with parchment paper and lightly grease the paper with shortening. This is VERY important to do or your bars are going to stick to the bottom of the pan. Reserve 1 cup oat mixture and press remainder onto bottom of the baking pan.
Bake 10 minutes
While the crust is cooking, chop the Halloween Oreos into pieces.
After removing from oven, immediately pour sweetened condensed milk evenly over crust.
Sprinkle with semi-sweet and white chocolate chips and top with Oreo pieces.
Top with reserved oat mixture; press down firmly.
Bake an additional 25 minutes. Topping will be lightly browned.
Cool. Cut into bars. Eat. FYI – these are amazing served warm and topped with vanilla ice cream.