In a large skillet, heat oil. Add the onion, peppers, and celery and cook for 5 to 6 minutes or until soft. Add the cayenne pepper, oregano, and salt, and cook for 2 minutes. Add the flour and cook for 1 minute, toasting the flour to a light brown color. Add the hot water and bring to a boil. Reduce the heat and simmer mixture for 10 minutes.
As sauce simmers, cook the pasta, timing pasta to finish cooking at the same time as the sauce. Add the crawfish to sauce, and continue simmering for 5 minutes. Add the green onions and cook for 1 minute more. Add cream and heat gently. Spoon crawfish mixture over orzo pasta in large pasta dishes.