Preheat the oven to 275 degrees . In a small bowl, whisk together the paprika, chili powder, garlic powder, oregano, 1 tablespoon salt and 1 teaspoon pepper. Run a paring knife down the length of the bone side of each rack to split the skin, then season all over with the rub.
Arrange the ribs, meaty side down, in a single layer on a baking sheet. Roast, turning halfway through, until very tender, about 3 hours.
Meanwhile, in a small saucepan, bring the tomato sauce, onion, vinegar, brown sugar and hot sauce to a boil. Lower the heat and simmer until thickened, about 10 minutes. Season with salt and pepper. Cut the racks into individual ribs and serve with the sauce on the side.