If you're serving right away you can add the orzo pasta and chicken at the same time until the orzo is cooked. If you plan on serving later it at a later time you may want to cook the orzo separately and add right before serving. The orzo pasta tends to soak up all the liquid and can become quite thick if you simmer the soup for a long time. You can always add more chicken stock if you want to thin it out a little.
Source: slightly adapted from Food.com