Preheat oven to 350 degrees.In a stand mixer with the paddle attachment cream butter and sugar until light and fluffy, about 3 minutes.
Add in peanut butter and mix for 1-2 minutes.
Add in vanilla and egg. Mix until combined.
Combine flour, salt and baking soda in a small bowl.
Add the dry ingredients to the batter in two parts with the stand mixer on the lowest speed. Mix for a little less than a minute. Do not over mix.
Fold in old fashioned oats, M & M's and chocolate chips.
Portion dough into 16-18 cookies and place on a silicone baking mat on top of a baking sheet.
Bake cookies for 11-13 minutes or until lightly golden brown and firm on the edges.Remove from the oven and let set for 3-4 minutes.
Transfer to a cooling rack to cool completely.
Cookies can be stored in an airtight container at room temperature for 3-5 days (if they last that long). You can always freeze the cookies for up to 3-4 months.