In a large serving bowl, combine the cucumbers, tomatoes, red onion, kalamata olives and parsley. Place the avocado in a small bowl and set aside.In a small canning jar, combine the olive oil, red wine vinegar, garlic, oregano, sugar and salt and pepper. Top with the lid and shake well until blended and emulsified. Season with more sugar and salt and pepper to taste.Pour 1 tablespoon or so of the dressing on the avocado and gently mix to coat. Pour the rest of the dressing on the cucumber mixture and toss to coat. Add the avocado to the salad and top with chunks of feta cheese, and serve.