Preheat the oven to 350° and line 2 baking sheets with parchment paper.
In the bowl or a stand mixer, cream together the butter and sugars until smooth, light and fluffy. This will take a few minutes. It's important to really get this mixture fluffy to have the best cookies.
Scrape the sides of the bowl with your spatula and mix in the egg, vanilla, and peanut butter.
Once completely combined, stop the mixer and add in the flour, rolled oats, baking soda, and salt.
Start mixing slowly and adding in your flour. Once the flour mixture is about half way incorporated into the dough, pour the M&M's and chocolate chips into the mixer and allow everything to mix together.
Stop the mixer as soon as the flour is completely incorporated. Do not over mix.
Measure out the dough into ⅓ cup portions of cookie dough for each cookie (I usually just do 1 cookie scooper amount). Form the dough together in a ball leaving the top jagged, for texture. Avoid packing the dough together tightly.
Bake at 350° for 9-11 minutes, or until the cookie dough has spread and the edges are turning golden brown.
Allow the cookies to cool on the pan for 20 minutes. Serve warm and enjoy!