Thoroughly mix flour, brown sugar, oats and cinnamon in a bowl.
Cut your stick of butter into smaller ½ tablespoon pieces and add to your dry ingredients.
If you have a food processor you can use that to pulse your mixture. You can also use a pastry knife to begin to cut the flour into the dry ingredients. When I get to a certain point, I start to press the butter into the mixture with the back of my hand balled into a fist, then pick up the chunks and further mix everything together with my fingers by pressing the dry ingredients into the butter.
Continue mixing your topping until you have a chunky texture.
Crumble topping evenly over the apples.
Bake for 45 minutes or until apples are soft but not mushy. I like to cover my dish after about 35 minutes so the topping doesn't get too brown. Serve warm with ice cream or whipped topping and a drizzling of caramel sauce.
Store uncovered on counter or covered in refrigerator for 3 days. Rewarm in microwave if desired.