Preheat your grill to medium-high heat.
In a food processor, combine the black olives, garlic, olive oil, dried oregano, salt, and black pepper. Blend until smooth.
Place the leg of lamb on a cutting board and use a sharp knife to make small cuts all over the surface of the meat.
In a mixing bowl, whisk together the olive purée and lemon juice.
Rub the olive purée mixture all over the leg of lamb, making sure to get it into all of the cuts.
Place the leg of lamb on the grill and cook for 15 minutes, turning occasionally.
After 15 minutes, baste the lamb with some of the remaining olive purée mixture and continue cooking for another 15 minutes, basting occasionally.
Check the internal temperature of the lamb with a meat thermometer. For medium-rare, it should be 135-140°F. Cook for a few more minutes if needed.
Remove the lamb from the grill and let it rest for 10 minutes.
Slice the lamb and serve with the remaining olive purée mixture and chopped herbs, if desired.