Preheat oven to 350 degrees F. Line cookie sheets with parchment paper.
Optional: Spread oats on a baking sheet and toast for 10 minutes. Set aside.
In the bowl of stand mixer fitted with the paddle attachment, or in a large mixing bowl using a hand mixer, beat butter and brown sugar on medium speed until light and fluffy, about 4-5 minutes.
Add egg and vanilla extract and beat until fluffy, about 3 minutes.
Add flour, baking powder, baking soda, salt, cinnamon, and milk to butter mixture. Mix on medium/low speed until a dough forms.
Add oats to mixture and mix on low speed until evenly incorporated. Add raisins to the cookie dough and mix until combined.
Use a cookie scoop to drop dough (about 2 ounces) onto prepared baking sheets. Make sure to leave enough space between cookies. You should have about 6 cookies per cookie sheet. Use your fingers to lightly press flatten the tops.
Bake cookies for 10-12 minutes, or until edges are golden brown and center looks a little bit undercooked.
Remove from oven and let cool 10 minutes on cookie sheet before transferring to a cooling rack to cool completely.