Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper to prevent sticking and ensure easy cleanup.
Cream together the butter and sugars in a large bowl until light and fluffy, about 2-3 minutes. This creates air pockets for a better texture, so don’t rush this step.
Beat in the eggs one at a time, then add the vanilla extract, mixing well after each addition until completely incorporated.
In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt until evenly combined.
Gradually add the dry ingredients to the wet mixture, stirring just until combined. Over-mixing can make your cookies tough.
Fold in the oats first, making sure they’re evenly distributed throughout the dough.
Gently fold in the diced apples and any optional add-ins (nuts or dried fruit) until just incorporated. The moisture from the apples will soften the dough slightly.
Drop rounded tablespoons of dough about 2 inches apart onto the prepared baking sheets. The cookies will spread a bit during baking.
Bake for 12-15 minutes until the edges are lightly golden but the centers still look slightly soft. They’ll continue cooking on the hot baking sheet.
Let the cookies cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.