Preheat the oven to 350°F and line a 9x13-inch pan with parchment paper and set aside.
In a large bowl, whisk together flour, salt, and baking soda, and set aside.
In the bowl of an electric mixer, cream together the butter and sugars until combined, then add eggs and vanilla and beat well, scraping down sides as needed.
Slowly add in flour mixture, and mix until just combined. Fold in 1 cup each of candy and chopped Oreos.
Spread batter evenly into prepared pan and top with remaining candies, cookies, and sprinkles.
Bake for 24 to 27 minutes until the edges are just golden. Let cool in pan completely before using the edges of the parchment to pull them out and cut them into squares.
Notes
Regular M&M's could be used if you can't find the Cookies & Scream ones. You could also use chopped Hershey's Cookies n' Cream bars.
Store cookie bars in an airtight container at room temperature for up to 3 days.
I used regular Oreos, but Double Stuf could also be used.