Preheat the oven to 350ºF (177ªC). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
In a large bowl, toss together the oats, flour, baking soda, cinnamon, and salt. Set aside.
In a medium bowl, whisk together the melted butter and brown sugar until smooth. Whisk in the egg, peanut butter, and vanilla until combined.
Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. Be patient-- dough is thick. Fold in the m&m's and chocolate chips. Allow the cookie dough to sit for about 10 minutes to help the oats absorb some of the moisture.
Using a cookie scoop, drop scoops of cookie dough onto the prepared baking sheet. Gently press down on the tops of each cookie dough ball to flatten the tops slightly.
Bake the cookies for 10 minutes until the edges look set. Centers will look very soft and underbaked. This is ok. Remove from the oven and allow to cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely. Cookies stay fresh in an airtight container at room temperature up to 1 week. Baked cookies may be frozen, up to 3 months. Rolled cookie dough can be frozen, up to 2 months. Roll dough balls and chill in the refrigerator until firm before freezing. Do not thaw and bake for an additional 1-2 minutes.
Quick oats: these are essential for these cookies. Old fashioned oats do not absorb liquid well enough. If all you have are old fashioned, pulse in a food processor until oats are fine.
m&m's: any small m&m's are fine (even mini). Just do not use larger m&m's (like peanut or peanut butter).