In a large bowl, whisk together flour, baking soda, salt, and cinnamon. Set aside.
In a large bowl using an electric mixer, cream together butter and sugars on medium speed. Add eggs and vanilla extract; beat well.
Add flour mixture to wet ingredients; mix until combined. Add oats; mix well. Optional: fold in add-in of your choice.
Scoop dough into heaping tablespoon balls or use a medium cookie scoop, #40 disher, and place on an ungreased baking sheet or silicone baking mat/parchment paper lined baking sheet.
Bake for 10-13 minutes, or until light golden brown.
Cool cookies on a baking sheet for a minute then transfer to a wire rack to cool completely. Eat & Enjoy!
Notes
Classic Oatmeal Cookies should be stored in an airtight container; will keep 5+ days.¹ A rounded teaspoon is a little more than if the teaspoon were leveled. ² Optional: Add 1 cup of your favorite add-in; i.e. raisins, walnuts, chocolate chips, etc.