Roughly chop the lettuce, tomatoes and cucumbers. You can chop them in any size you like, but traditionally the vegetables in a Greek salad should be chopped coarsely. Mix the vegetables in a deep salad bowl.
Cut the cheese into large cubes. The onion for the salad should also be chopped into large rings.
Mix olive oil, salt, pepper, oregano and lemon juice in a bowl. Pour the resulting sauce over the vegetables and add olives in a random order. Mix the salad before serving.