Combine all of the ingredients for the filling in a bowl and stir to coat. If using frozen cranberries, do not thaw.
Pour the filling into a 9" round tart pan.
In the same bowl, combine all of the ingredients for the topping except the rolled oats.
Using a pastry cutter, cut the cold butter into the dry ingredients until you're left with pea-size chunks. There should be no dry ingredients at the bottom of the bowl. If there are, gently use your fingers to rub it into the butter.
Add the oats and stir to coat.
Pour the topping over the filling.
Bake at 350F for 40-50 minutes or until a thick syrup bubbles up around the edges.
Notes
Estimating apples: You'll need two to three medium size apples to create three cups of cubed apples.
Pastry cutter substitute: If you don't have a pastry cutter on hand, you can cut the butter into the dry ingredients using a fork or by rubbing it together with your fingertips.