Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cinnamon, baking soda, and salt.
In a large bowl, cream the softened butter with the brown sugar and granulated sugar until light and fluffy.
Add the egg and vanilla extract to the butter mixture and beat until well combined.
Gradually stir the dry ingredients into the wet ingredients until just incorporated.
Fold in the rolled oats and diced apples until evenly distributed throughout the dough.
Drop rounded tablespoons of dough onto the prepared baking sheets, spaced 2 inches apart.
Bake for 10 to 12 minutes or until the edges are lightly browned and the centers are set.
Cool on the baking sheet for 5 minutes before transferring to a wire rack for complete cooling.