Preheat the oven: Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper or silicone mats. Cream butter and sugars: In a large bowl, beat the butter, peanut butter, granulated sugar, and brown sugar together until light and fluffy. Add eggs and vanilla: Beat in the eggs, one at a time, then mix in the vanilla extract. Mix dry ingredients: In a separate bowl, whisk together oats, flour, baking soda, and salt. Gradually add the dry mixture to the wet ingredients, mixing until combined. Add mix-ins: Stir in the chocolate chips and M&M’s until evenly distributed. Shape the cookies: Scoop about 2 tablespoons of dough (or use a medium cookie scoop) for each cookie. Roll into balls and place on the prepared baking sheets, spacing them about 2 inches apart. Press a few extra M&M’s on top of each cookie for decoration. Bake: Bake for 10-12 minutes, or until the edges are lightly golden and the centers look set but slightly soft. Cool: Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.