Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, oats, baking powder, baking soda, cinnamon, and salt.
In another bowl, mix the Greek yogurt, brown sugar, egg, and vanilla extract until well combined.
Add the wet mixture to the dry ingredients and stir until just combined; do not overmix.
Fold in the diced apples until evenly incorporated.
Spoon the batter into the prepared muffin tin, filling each liner about 2/3 full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring to a wire rack.
Enjoy your Apple Cinnamon Greek Yogurt Muffins warm or at room temperature!