Combine the lettuce, tomatoes, cucumber, and onion in a large bowl. Pour enough olive oil over the salad to just coat it. Add the lemon juice and oregano. Season well with salt and pepper and toss the salad. Adjust the seasoning to suit your taste and toss again.
Add the crumbled feta cheese and the olives to the top of the salad before serving.
Serves 4 to 6 as a side salad, 2 or 3 as a lunch portion salad.