Step 1: Preheat oven to 350 degrees. In a large bowl or electric mixer, blend butter, sugar, and brown sugar. Add eggs and vanilla and beat until creamy and smooth.
Step 2: In a separate bowl, whisk together flour, baking soda, salt, and cinnamon.
Step 3: Add this dry mixture to the butter/sugar mixture and blend well. Mix in the oats.
Step 4: Peel and finely dice the apple. You should get about 1 cup of apple pieces.
Step 5: Stir the apple pieces into the dough.
Step 6: Roll dough into 1-inch balls and place evenly on a lined cookie sheet.
Step 7: Bake for 12 minutes or until lightly browning on the edges of the cookies. Let cool for 5 minutes on the baking sheet and then transfer to a cooling rack.