Preheat oven to 400 degrees F (200 degrees C). Grease six muffin cups or line with paper muffin liners.
Stir together 1 1/2 cup flour, 3/4 cup sugar, salt, baking powder and 1 teaspoon cinnamon. Mix in oil, egg and milk. Fold in apples. Spoon batter into prepared muffin cups, filling to the top of the cup.
In a small bowl, stir together 1/2 cup sugar, 1/3 cup flour, butter and 1 1/2 teaspoons cinnamon. Mix together with fork and sprinkle over unbaked muffins.
Bake in preheated oven for 20 to 25 minutes, until a toothpick inserted into center of a muffin comes out clean.