With a sharp paring knife, cut 12 slits just under the fat of the roast and insert the garlic slices. Season the roast with the salt, pepper, and oregano. Place it, fat-side down, on a roasting rack. Place rack in a roasting pan and fill with water, just skimming the bottom of the roast. Heat grill to medium high. If using a charcoal grill, build a very hot fire. When the coals are white hot, add another layer of charcoal. This will ensure the grill stays hot enough to cook the meat. Place pan directly on grill. Cover grill and allow the roast to cook slowly. Cook about 20 minutes per pound for medium rare or until a meat thermometer reads about 130° F. Halfway through, turn the roast and add more water if needed. Let rest about 10 minutes before carving.