Preheat oven to 175°C | 350°F.
Arrange the sliced apples into a 9x13-inch baking dish.
Add the sugar, flour and cinnamon over the apple slices.
Pour in the water and mix everything through until apples are evenly coated.
In a bowl, combine the oats, flour, brown sugar, cinnamon, baking powder and melted butter.
Mix well using a metal spoon or your fingertips, until the butter has soaked through the dry ingredients (about ne minute of mixing).
Sprinkle evenly over the apple filling.
Bake in preheated oven for 40 - 45 minutes, or until he crumb is golden and crisp and the apples are cooked to your liking.