Preheat oven to 350º F (180º C). Line a large baking sheet with parchment paper or a silicone baking mat.
Combine flour, oats, baking powder, baking soda, and salt in a bowl. Set aside.
Using an electric hand mixer or a stand mixer, beat the cold butter on medium speed for 30 to 45 seconds just to break it down a little. Add the granulated sugar and brown sugar and mix until just combined, 30 to 45 seconds. Add peanut butter and beat until incorporated. Scrape down the sides and bottom of the bowl as needed.
Reduce the speed to low and add the flour mixture; mix until it resembles coarse crumbs. Add the milk and vanilla; mix until the dough comes together into a big ball.
Add the M&M's and chocolate chips and gently fold them into the cookie dough.
Use a small cookie scoop (1.5 tablespoons) to measure out the oatmeal cookie dough. Roll each portion into a ball and place them evenly spaced, about 2 inches apart, onto the prepared cookie sheet.
Bake for 10-12 minutes, until barely golden brown around the edges but still soft in the middle. When you remove the cookies from the oven, they will still look doughy, and that is okay; they will continue to set as they cool. Note: The cookies spread just slightly while baking, so press them down with a spoon after you remove them from the oven. That’s what gives the cookies their crinkled tops.
Cool on baking sheets for 2 minutes; remove to wire racks to cool completely.