In a medium bowl, whisk together flour, ground cinnamon, baking soda, baking powder and salt and set aside.
In the bowl of a stand mixer or using a large bowl and a hand mixer, beat the butter on medium-high speed until smooth, about 1-2 minutes. Add the brown and granulated sugar, then continue to beat until light and fluffy.
Keep the mixer running as you add the eggs and vanilla. Be sure to scrape down the sides and bottom of the bowl and mix again to make sure everything is combined.
Gradually add the dry ingredients to the wet ingredients and mix on a low speed until combined. With the mixer still running, add the oats in one cup at a time. Add in the chocolate chips or chunks. Once done, the dough will be thick and sticky. Cover and refrigerate for an hour or even 2 hours is better.
Preheat the oven to 350 degrees. Line two baking sheets with parchment paper. Set aside.
Use an ice cream scooper to scoop out the dough directly onto the baking sheets. Each sheet will fit 6 large cookies or 12 smaller ones.
Bake for 12-14 minutes (8-10 if small), or until the edges are lightly browned but the middles are still very soft.
Remove the cookies from the oven and allow them to cool off on the baking sheet for 5 minutes. Transfer them onto a wire rack to let them completely cool.