Heat broiler. Peel shrimp, leaving tails intact; remove veins from backs. Place shrimp in a small bowl, and toss with 2 tablespoons olive oil, garlic, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Thread three shrimp onto a metal skewer; place on a baking sheet. Repeat with remaining shrimp and three more skewers. Broil until tops are browned, 2 to 3 minutes. Flip; cook until browned and cooked through, 2 to 3 minutes more. Set shrimp aside.
In a bowl, whisk together lemon juice, oregano, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper. Whisking constantly, slowly drizzle in remaining A cup olive oil. Set aside.
Divide romaine among four plates. Place some of the tomato, cucumbers, onion, and olives on each plate; top with a reserved shrimp skewer. Drizzle with reserved dressing, and serve.