Prepare orzo according to package directions; drain and rinse under cold water. Drain again very thoroughly. Place in a large bowl.
Stir in the zucchini, olives, green onions, celery, tomato, and green bell pepper.
In a small bowl, beat together the olive oil, vinegar, garlic, oregano, dill, salt and pepper.
Pour over the salad and toss to coat well.
Sprinkle on the feta cheese and toss again. Add the mayonnaise and toss gently.
Chill at least for 2 hours and bring to room temperature before serving.