Preheat oven to 350°F. Grease a 12 cup muffin tin with butter or oil. Alternatively, line the pan with muffin liners.
In a large bowl, combine the flour, oats, sugar, flaxseed, baking soda, baking powder, salt, and cinnamon. Stir together, breaking up any clumps of brown sugar with the back of your spoon or spatula.
Add the egg, buttermilk, water, and oil to the dry ingredients and gently stir until just combined. Fold in the chopped apple, being careful not to over-mix.
Using two spoons, scoop the batter into the prepared muffin cups, dividing it evenly among the 12 cups. Bake muffins for about 25 minutes or until a toothpick inserted into the centre comes out clean. Let cool for 5-10 minutes in the pan, then remove the muffins and let them cool completely on a wire rack.