Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large mixing bowl, cream together the butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt. Gradually add to the butter mixture and mix well.
Stir in the oats and diced apple until evenly distributed.
Drop by rounded tablespoonfuls onto the prepared baking sheet.
Bake for 10-12 minutes, or until the edges are golden brown. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a cooling rack.