Lay the prime rib on the cutting board. You want a good cap of fat left on the meat, 1/4 to 1/2 inch, can trim off excess.
Rub the entire surface with cooking oil.
Sprinkle generously with salt and pepper.
If you have the time, it is ideal to place it uncovered in the fridge for 24 hours. This allows the salt to penetrate ans is called a dry brine.
Take it out of the fridge about an hour before cooking to take the chill off.
Prepare you smoker and set the temperature at 225 degrees.
Place the rib roast directly on the cooking rack bones down. Insert a meat thermometer so you can determine when to take it out.
Add your chips or pellets for the smoke. Cherry, apple or any lighter flavored wood is good. Smoke it for at least the first 2 to 3 hours.
Continue cooking in the smoker until the internal temperature is 135 to 140 for medium rare. Total cooking time can take from 5 to 7 hours depending on the size of roast/number of bones.
Remove to a platter and immediately tent with foil and let rest for 30 minutes. Longer if it's a really big one.
While it's resting mix everything for the horseradish sauce, taste and adjust. Some people like a squeeze of lemon juice or dash of Worcestershire. Make it yours.
Slice between the bones and serve. Sometimes I cut the meat from the bones in one piece so that I can slice it thinner. Then I slice the bones apart and put them on the platter and the family fights over them!