In a medium-size bowl, whisk together flour, old fashioned oats, and baking soda. Set aside.
In a large bowl using an electric mixer, cream together butter and sugars on medium speed; until light and fluffy. Add peanut butter, vanilla extract, and egg; mix until combined.
Scoop dough into 2 teaspoon balls, or use a #40 disher to scoop balls of dough. Place dough on an ungreased baking sheet or silicone mat/parchment paper-lined baking sheet. Slightly flatten dough balls with the bottom of a cup and add a few M&M's candies on top.
Bake 12-14 minutes or until barely golden brown around the edges. Do not overbake. The tops will not brown. Cookies may appear to be soft and undone, but they will set as they cool.
Cool cookies on baking sheet for 5 minutes; transfer to a wire rack to cool completely. Eat & Enjoy!
Notes
Monster Cookies should be stored in an airtight container and will keep 4+ days.