Combine olive oil, lemon juice, mustard, garlic, oregano and black pepper in a resealable plastic bag. Place lamb in bag, seal and refrigerate 6 to 24 hours.
Preheat broiler (see Note for grilling method). Drain lamb, reserving any marinade. Place lamb on roasting pan.
Broil 6 inches from heat, 15 minutes on each side. Tent foil over lamb and continue cooking 10 to 15 more minutes or until instant-read meat thermometer registers 130°F for medium rare, 140°F for medium. (Lamb should not be served well done.)
To grill, place lamb over hot coals and sear, turning once, until well browned on both sides, about 10 minutes total. Lower heat to medium or move lamb to cooler edge of grill; cook 15 to 20 minutes longer, or until its the way you like it. Baste meat once or twice with reserved marinade. Remove lamb; let stand, covered with foil, for 10 minutes. Then slice. The lamb can also be roasted in a 425°F oven for 30 to 40 minutes.