Heat oven to 400 F. In a large bowl, combine the zucchini, onion, bell peppers, mushrooms, parsley, garlic, lemon juice, olive oil, oregano, black pepper and salt. Set aside for 10 minutes to marinate. Lightly coat a 10-by-15-inch pan with cooking spray. Arrange vegetables on the pan in a single layer. Roast in the oven for 20 minutes or until vegetables are crisp-tender. Meanwhile, in a medium saucepan, heat the orzo and oil on medium heat. Stir until the pasta is lightly browned. Add the chicken stock; stir frequently. Once the orzo is soft and tender, remove from heat and drain. Add the roasted vegetables to the orzo.