Create other versions of this salad by adding crumbled feta or goat cheese, Kalamata olives, or sliced cucumbers.
Layer tomato and onion slices on a serving platter. Sprinkle evenly with chopped fresh oregano.
Whisk together oil, vinegar, salt, and pepper. Drizzle evenly over tomato and onion slices. Let stand 10 minutes before serving.