In a small bowl, whisk all dressing ingredients together. Taste test, then adjust to your preferences.
Using a very sharp knife, carefully cut the romaine head in half lengthwise through the base to keep it's nice shape intact (as shown).
Set your lettuce aside on a platter, cut side up.
Top with tomatoes, cucumber, onions, olives, and feta cheese.
Drizzle with your homemade Greek yogurt dressing and sprinkle with dried oregano.
Serve immediately and enjoy!