Remove prime rib from refrigerator and let stand at room temperature for 2 hours before cooking.
Preheat oven to 450°F. In a small bowl, mix together salt, black pepper, garlic powder, onion powder, thyme, rosemary, and cayenne pepper.
Rub the entire roast with olive oil, then coat generously with the seasoning mixture, pressing it into the meat.
Place the roast bone-side down in a roasting pan and cook at 450°F for 15 minutes to create a crust.
Reduce oven temperature to 325°F and continue roasting for 13-15 minutes per pound for medium-rare (about 1.5-2 hours total), until internal temperature reaches 120-125°F.
Remove from oven, tent loosely with foil, and let rest for 20-30 minutes before carving. The temperature will rise to 130-135°F during resting.
Slice between the bones and serve immediately with horseradish sauce.