Preheat broiler and lightly oil a flameproof 3-quart shallow baking dish (2 inches deep).
Cook orzo in a 6-quart pot of boiling salted water until al dente. Reserve 3/4 cup pasta cooking water, then drain orzo in a colander. Toss hot orzo with remaining ingredients, reserved cooking water, and salt and pepper to taste in a large bowl until cheese is melted.
Transfer orzo mixture to baking dish and broil 2 inches from heat until lightly browned, about 4 minutes.
