Preheat oven to 350 degrees F. Grease a 9x13-inch pan generously with nonstick cooking spray.
In a large bowl, whisk together the flour, oats, baking soda, and salt. Set aside.
In the bowl of a stand mixer, cream the butter, peanut butter, and sugars together until smooth, about 2 minutes, scraping down the side of the bowl with a spatula, if necessary. Add in the eggs and vanilla and mix until combined.
Add the dry ingredients and mix until just combined, don’t over mix.
Stir in the chocolate chips and 1 cup of M&M’s.
Use a spatula to spread the mixture into the prepared pan. Top with the remaining 1/2 cup of M&M’s.
Bake for 25-35 minutes or until bars are lightly golden brown and set. Don't over bake. You don't want the bars to be dry. Remove the pan from the oven and sprinkle with flaky sea salt. Let the bars cool in the pan. When cool, cut Into bars and serve.
If you want to make the dough in advance, you can chill the dough for up to 48 hours. Chilling will also soften the oats.