Preheat the oven to 375 degrees and grease a 9×13 pan. When you slice the apples, I recommend slicing them into various widths for texture. Layer the slices into the pan.
In a medium bowl, combine the brown sugar, oatmeal, flour, cinnamon, and vanilla. Once mixed, cut in the butter using a pastry blender or fork until the mixture forms large crumbs.
Spread the mixture out on top of the apples and then bake for 30 minutes. Serve alone or with some vanilla ice cream.