I think the lemon juice and the oregano are both very characteristically Greek things to have in a salad dressing. I think that's the ticket, with your olive oil and whatever else (see gaijingirl's post).

Tzatziki's good too, though! At my favorite Greek restaurant they have a gyros salad, that's a big pile of salad, with tzatziki on it with the meat on top, and the dressing on the side. So, so good!