Combine flour, baking powder, baking soda, cinnamon, and salt in a bowl. Set aside.
Beat butter, granulated sugar, and brown sugar in a large mixer bowl over medium speed until smooth and creamy, about 3 - 4 minutes. Scrape down the sides and bottom of the bowl as needed.
Add milk and vanilla; mix until incorporated. Reduce speed to low and gradually beat in flour mixture until combined. Stir in oats until evenly distributed.
Use a small cookie scoop (1.5 to 2 TBS) to measure out the oatmeal cookie dough. Roll each portion into a smooth ball and place it on a cookie sheet or large plate. Refrigerate for at least 30 minutes up to 2 days.
Preheat oven to 350º F (180º C). Line a large baking sheet with parchment paper or a silicone baking mat.
Place the cooled cookies balls evenly spaced, about 2 inches apart, on your 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.
Cool on baking sheets for 2 minutes; remove to wire racks to cool completely.