1. Marinate the chicken by combining Greek yogurt, lemon juice, olive oil, oregano, garlic, and spices in a bowl.
2. Coat the chicken with the marinade and let it sit for at least 30 minutes.
3. Preheat a grill or pan and cook the marinated chicken until fully cooked.
4. Let the chicken rest before slicing it into bite-sized pieces.
5. Chop the romaine lettuce, cucumbers, tomatoes, red onions, and any additional vegetables.
6. Prepare the dressing by whisking together olive oil, lemon juice, red wine vinegar, oregano, garlic, and Dijon mustard.
7. In a large bowl, combine the chopped vegetables, crumbled feta cheese, kalamata olives, and sliced chicken.
8. Drizzle the dressing over the salad and toss everything gently.
9. Garnish with fresh herbs like parsley, mint, or more oregano.
10. Adjust the seasoning with salt and pepper as needed.
11. Serve the oregano chicken salad as a main dish or with pita bread on the side.
12. Store any leftovers in an airtight container for later use.