Using a vegetable peeler, peel half the lemon (avoiding bitter white pith); thinly slice peel. Squeeze 2 tablespoons juice; set aside.
Bring a large saucepan of salted water to a boil. Add orzo and lemon peel. Cook until orzo is al dente, according to package instructions. Drain and return to pot.
Add lemon juice, oil, and scallions. Season with salt and pepper; toss to combine.