In a very large mixing bowl, toss together the apples, cranberries, sugar, apple juice, and apple pie spice. Pour mixture into a 6-quart electric or stovetop pressure cooker.
For topping: In a medium bowl, stir together oats, brown sugar, flour, and cinnamon. Using a pastry blender or fork, cut in butter until mixture resembles coarse crumbs. Sprinkle topping over fruit mixture.
Lock lid in place. Set electric cooker on high pressure to cook 8 minutes. For stovetop cooker, bring up to pressure over medium-high heat according to manufacturer’s directions; reduce heat enough to maintain steady but not excessive pressure. Cook 8 minutes. Remove from heat.
For electric and stovetop models, let stand to release pressure naturally, at least 15 minutes, or according to manufacturer’s directions. If necessary, quickly release any remaining pressure according to manufacturer’s directions. Open lid carefully. Let stand 10 minutes. Serve warm with ice cream, if desired.