Rinse chicken and cook in pan with a little water for about 5-6 minutes until thoroughly cooked. Remove from pan and let cool; then shred. Rinse and dry salad greens, rinse and chop tomato and slice olives. Whisk together oregano, garlic powder, oil and vinegar to make dressing. Top greens with shredded chicken, tomato and olives. Pour dressing over salad, sprinkle with crumbled feta cheese and toss. Serve with toasted pita bread.