In a large bowl, whisk the flour and baking soda together. Add the oats and stir until the ingredients are evenly combined.

In the bowl of an electric mixer fitted with the paddle attachment, cream the butter until smooth and pale in color. Add the sugars and mix on low speed until just incorporated. Do not overmix.

Scrape down the bowl and add the eggs, one at a time, beating until smooth and scraping down the bowl after each addition. Add the corn syrup and vanilla and beat until just incorporated.

Scrape down the bowl and add the peanut butter. Mix on low speed until just combined. Add the oat mixture in three additions, mixing on low speed until just incorporated.

Use a spatula or wooden spoon to fold in the chocolate chips and M&Ms. Cover the bowl tightly and refrigerate for 5 hours.

Preheat the oven to 375ºF. Line a baking sheet with parchment paper or a silpat.

Spoon dough onto prepared baking sheet. Scoop 1 tablespoon for small cookies or 2 tablespoons of dough for large cookies. Place cookies 2 inches apart. Bake for 11-15 minutes until the cookies just begin to brown. Let cool on the pans for 8 to 10 minutes before transferring the cookies to a wire rack to cool completely. Cookies can be stored in an airtight container for up to 3 days.