Preheat oven to 350 degrees. Line two light-colored baking sheets with parchment paper and set aside.
In a large bowl, cream butter, peanut butter, sugar, and granulated sugar together until light and fluffy. Stir in eggs and vanilla. Scrape the sides and stir again briefly.
Stir in flour, baking soda, salt, and oats until just incorporated. Scrape sides and bottom of the bowl and stir again briefly. Stir in M&M's by hand.
Scoop cookie dough into large 3-tablespoon mounds and place onto baking sheets. Bake 8-9 minutes or until edges are lightly golden and cookies are puffed. Cool 3-4 minutes on the hot baking sheet before transferring to a cooling rack.
If you are going to use all natural peanut butter, be sure to reduce the flour amount by 1/4 cup.