Cut the tomatoes in half lengthwise, cut out the core, and cut each half into 4 wedges. Put the tomatoes into a large salad bowl, and add the cucumber, feta cheese, onions and olives.
Spoon over the olive oil and lemon juice. Season with sea-salt and freshly ground black pepper, bearing in mind that the feta is very salty. Toss gently to mix. Sprinkle the salad with fresh oregano and serve immediately.