Preheat oven to 400°F (200°C) and grease a 12-cup muffin tin or line with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, and salt.
In a separate measuring cup, combine the vegetable oil, egg, and milk.
Pour the liquid ingredients into the dry ingredients and stir with a spatula until just combined.
Gently fold the diced apples into the batter.
Divide the batter evenly among the 12 muffin cups.
Bake for 20 to 25 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Transfer the muffins to a wire rack to cool completely.