Soak 16 wooden skewers in water. Heat grill to medium-high.
Thread the lamb onto the skewers. Brush with 1 tablespoon of the oil and season with the oregano, 1 teaspoon salt, and ½ teaspoon pepper.
In a large bowl, toss the bell peppers and onions with the remaining 2 tablespoons oil, ½ teaspoon salt, and ¼ teaspoon pepper.
Grill the lamb and vegetables, turning frequently and brushing with the lemon juice, until the lamb is medium-rare and the vegetables are tender, 8 to 10 minutes.
