Cook the orzo: Bring a pot of salted water to a boil. Add orzo and cook according to package instructions (usually 7–9 minutes). Drain and rinse with cold water to stop the cooking. Let it cool completely.
Prepare the chicken: If not using pre-cooked chicken, grill or sauté chicken breasts seasoned with salt, pepper, and oregano. Once cooked through, let rest for 5 minutes, then chop into bite-sized pieces.
Make the dressing: In a small bowl, whisk together olive oil, vinegar, lemon juice, mustard, garlic, oregano, salt, and pepper until emulsified.
Assemble the salad: In a large mixing bowl, combine the cooked orzo, chicken, cherry tomatoes, cucumber, red onion, olives, and parsley. Drizzle the dressing over and toss to coat.
Add feta: Gently fold in crumbled feta cheese.
Chill & serve: For best flavor, refrigerate for at least 30 minutes before serving. Garnish with extra parsley and feta if desired.