1Heat coals or gas grill for direct heat. Peel onions; cut 1/2-inch slice from top of each onion and leave root end. Cut each onion from top into 8 wedges to within 1/2 inch of root end. Gently pull wedges apart.
2Brush four 12-inch squares of heavy-duty aluminum foil with vegetable oil. Place 1 onion on each square; loosely shape foil around onion. Sprinkle onions with vinegar, oregano, brown sugar, salt and pepper. Wrap foil securely around onions.
3Cover and grill onions 4 inches from medium heat 50 to 60 minutes or until very tender. To serve, sprinkle croutons over onions.