The uniform thickness of a butterflied boneless leg of lamb makes it extremely simple to cook and serve. This cut of meat shouldn't be hard to find, but if you get one that is boned and tied, ask your butcher to butterfly it.
Combine oil, lemon juice, garlic, oregano, salt, and pepper in sealable plastic bag. Add lamb and seal bag, pressing out air. Turn bag to coat lamb, then put bag in a shallow baking pan and marinate, chilled, turning bag over occasionally, at least 8 hours.
Bring lamb to room temperature, about 1 hour, before grilling.
Prepare grill for cooking over direct heat with medium-hot charcoal (moderate heat for gas);
Remove lamb from marinade (discard marinade) and run 3 or 4 skewers lengthwise through lamb about 2 inches apart. Grill on a lightly oiled grill rack, covered only if using gas grill, turning over occasionally and, if necessary, moving around on grill to avoid flare-ups, until thermometer registers 125 to 128°F, 8 to 14 minutes total for medium-rare.
Transfer lamb to a cutting board and remove skewers. Let lamb stand, loosely covered with foil, 20 minutes. (Internal temperature will rise to 135°F while meat stands.) Cut across the grain into slices.
If you aren't able to grill outdoors, lamb can be cooked in a hot lightly oiled well-seasoned large (2-burner) ridged grill pan, uncovered, turning over once, 12 to 14 minutes per side. · Lamb can marinate up to 24 hours.