Preheat the oven to 350°F. Line two baking sheets with parchment paper or Silpat baking mats.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the sugar, brown sugar, and butter until light and fluffy.
Beat in the eggs, one at a time, and then beat in the peanut butter, vanilla, baking soda, and salt until well combined.
Add the oats and mix until combined, then add the mini M&Ms and mini-chocolate chips.
Scoop 2-to 3-tablespoon mounds of the dough onto the prepared baking sheets, spacing the mounds at least 2 inches apart.
Bake the cookies for 12-15 minutes then remove them from the oven and allow them to cool for 5 minutes on the baking sheets before transferring them to a rack to cool completely.
The best way to guarantee your cookies are chewy and not too crispy is to underbake them slightly then remove them from the oven and allow them to cool for 5 to 10 minutes on the baking sheets.