Pre-heat the oven to 350°F and line a 13x9-inch metal baking pan with greased parchment paper with a 2-inch overhang on each short side of the pan.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the sugar, brown sugar and butter until light and fluffy, about 2 minutes.
Add the eggs to the bowl, one at a time, beating between each addition then add the peanut butter, vanilla extract, baking soda and salt and mix until well combined.
Add the oats and mix just until combined. Stir in the M&Ms and chocolate chips.
Transfer the cookie dough into the prepared pan, pressing it into an even layer. Bake for 25 to 30 minutes until a toothpick inserted comes out clean. (See Kelly's Notes.) Remove the pan from the oven and allow the cookie to cool in the pan for 1 hour. Using the parchment paper overhang, lift the cookie out of the pan and onto a cutting board. Slice the cookie into 24 bars and serve.