In a medium bowl, add flour, salt, and baking soda and whisk together. Set aside.
Preheat oven to 350°F. Line a 9x9 inch pan with parchment paper.
In a large bowl or the bowl of your stand mixer with paddle attached, beat together butter, granulated and brown sugar for about 2-3 minutes.
Add eggs in 1 at a time, combining into the batter well.
Add vanilla and peanut butter and incorporate in.
Add oats and mix on low until they are well mixed into cookie dough.
Add your dry ingredients (flour, salt, baking soda) to your wet and mix in until just combined.
Add your m&m's, chocolate chips, and walnuts and stir in with a spatula.
Transfer cookie dough to parchment lined pan and smooth into an even layer.
Bake your cookie bars on the center rack of your oven for about 25 minutes.
Let your cookie bars cool in the pan for about 10 minutes, then use parchment to transfer to a cooling rack. Allow to cool before slicing.
Cookie bars can be stored in an airtight container for 3-4 days. Enjoy!
