Preheat the oven to 350°F. Line a baking sheet with parchment paper or coat with nonstick cooking spray.
In a medium bowl, mix together 1½ cups all-purpose flour, 1 tsp ground cinnamon, ¼ tsp salt, ½ tsp baking powder, ½ tsp baking soda. Set aside.
In a large bowl, cream 1 cup unsalted softened butter with a hand mixer for 1 minute. Next add 1 cup brown sugar and ½ cup granulated sugar. Mix until it’s light and fluffy, about 3 minutes.
Add 1 tsp vanilla extract and the 2 large eggs. Mix only until it’s combined.
Pour the dry ingredients into the wet ingredients bowl. Use the hand mixer to combine ingredients, but do not overmix.
Add the 3 cups rolled oats to the bowl. Use a spatula to fold the oats into the batter evenly.
Drop 2 Tbsp of cookie dough onto the prepared baking sheet. Each cookie should be 2″ apart to allow spreading during the baking process. Bake for 11-13 minutes, until the edges are golden brown.
Remove from oven and let cool for 5 minutes before using a spatula to transfer to a wire rack to cool completely. The cookies will be soft during this step but harden slightly as they cool.