Preheat oven to 350°F (170°C. Line baking sheets with parchment paper.
Beat butter and brown sugar together in a large bowl until creamy. Add peanut butter, eggs and vanilla and mix until incorporated. Stir in flour, baking soda and salt. Stir in oats, chocolate chips and M&Ms.
With a regular-sized ice cream scoop, scoop out dough and shape into balls. Place the balls on the baking sheets and flatten somewhat with your fingers.
Bake cookies for 10-12 minutes, just until the edges start to brown and/or tiny cracks appear on the surface. Remove from the oven and allow cookies to stand for 5 minutes before removing them to wire racks to cool completely.
In airtight containers, cookies stay fresh for one week at room temperature or keep for two months in the freezer.