Preheat oven to 375 degrees. Generously butter an 8 inch square baking pan.
Place the peeled, cored and sliced apples into your prepared pan.
In a medium size bowl combine the brown sugar, oats, flour and cinnamon.Cut the butter into the mixture with a pastry blender until you have coarse crumbs. (Sometimes I find rubbing the mixture between your fingers works just as well as a pastry blender.)
Sprinkle the topping over the apples. Bake 25 - 35 minutes or until apples are tender. Test by sticking a fork down into the crisp, you will feel a slight resistance if the apples aren't tender. Bake an additional 10 - 15 minutes until apples are tender. Serve warm with a scoop of ice cream