Preheat oven to 350F with rack on lower middle position. Lightly grease a 9x13" glass baking pan; set aside.
In the bowl of a stand mixer fitted with paddle attachment, mix together both sugars and eggs on medium speed, until well combined. Add cinnamon, salt, vanilla, peanut butter, and butter; mix until fully incorporated. Using a wooden spatula, stir in the chocolate chips, baking soda, and oats. Stir to combine well. Scrape dough into prepared pan and lightly/evenly press to fill all corners. Lightly press M&M's evenly across top of dough.
Bake 20 minutes or just until sides are golden brown. Do not over-bake. Bars will continue to set firmly upon cooling. Cool in pan, on wire rack completely. Cover and chill for ease of cutting. Cut chilled bars into squares and serve.
Notes
Baked bars can be wrapped tightly and frozen for 1-2 weeks.