Remove ribs from packaging and pat dry with paper towels. Cut each rack in half. Arrange ribs bone side up. Using a flexible knife, loosen thin layer of membrane from one of the half-racks of ribs. Using a dry paper towel for grip, pull membrane from ribs until it comes off and discard. Repeat with remaining racks.
Season ribs on both sides with salt. In a small bowl, stir cayenne, oregano, garlic powder, paprika, onion powder, and black pepper. Sprinkle seasoning mix on both sides of ribs.
Transfer ribs to a 6- to 7-quart slow cooker, arranging so bones are parallel to long sides of slow cooker. Pour 1 ½ cups BBQ sauce over ribs. Using a pastry brush, make sure sauce coats front and back of ribs.
Cook ribs until very tender, on high for 3 ½ hours or low for 7 hours.
Preheat oven to 400º. Transfer ribs bone side down to a large foil-lined baking sheet. In a small bowl, whisk ¼ cup leftover cooking liquid and remaining ¼ cup BBQ sauce. Brush sauce mixture over ribs.
Bake ribs until sauce begins to brown and caramelize, about 10 minutes. Transfer to a cutting board and let cool slightly before slicing.
