Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large mixing bowl, combine flour, sugar, baking powder, baking soda, salt, and cinnamon.

In another bowl, cream together softened butter and brown sugar until light and fluffy, about 3-4 minutes.

Add eggs one at a time to the creamed mixture, beating well after each addition. Mix in vanilla extract.

Gradually add the dry mixture to the butter mixture, alternating with buttermilk, mixing just until combined.

Gently fold in chopped apples and oats until evenly distributed.

Pour the batter into the prepared baking pan and smooth the surface.

Bake for 45-50 minutes or until golden brown and a toothpick comes out clean.

Cool in the pan for 10-15 minutes, then transfer to a wire rack to cool completely.