Preheat oven to 350 degrees F. Spray a 9x13 baking dish with cooking spray. 

In a separate bowl, combine the dry ingredients: flour, oats, salt, and baking soda. Mix and set aside. 

In the bowl of an electric mixer, combine the butter and peanut butter. Add in the brown sugar, and cream until mixture is light in color. 

Add the eggs and vanilla to the cookie dough, mixing between each addition. Scrape down the sides of the bowl as needed. 

Gradually add the dry ingredients until fully incorporated. 

Mix the candies, chocolate chips, and candy bar pieces into the cookie dough. (Reserve some pieces for the top, if desired.)

Turn the cookie dough out into the prepared 9x13 baking dish, and press down to flatten the top. Gently press the candy eyeballs into the dough. 

Bake cookie bars at 350 degrees F for 20-25 minutes, or until set. Cool completely, slice into bars, and enjoy!