Preheat oven to 325°. Place lamb on a rack in a shallow roasting pan, fat side up. Cut 12-14 slits 1/2-in. deep in roast. Combine garlic, thyme, marjoram, oregano, salt and pepper; spoon 2 teaspoons into the slits. Brush roast with oil; rub with remaining herb mixture.
Bake, uncovered, until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°), 2 to 2-1/2 hours. Let meat stand 15 minutes before slicing.