For the salad, drain olives well. Peel 2 garlic cloves and chop finely. Mix olives, chopped garlic, olive oil and oregano in a bowl, cover and marinate at least 24 hours.
Rinse cucumber, peel and cut into slices. Rinse tomatoes, core and cut into bite-size pieces. Peel onion and cut into thin rings.
For the dressing, mix lemon juice with 1/2 teaspoon salt and plenty of freshly ground pepper. Drain olives and add drained olive oil to dressing. Add additional olive oil as necessary to achieve desired consistency.
Cut remaining garlic clove and half and rub along the interior of a salad bowl. Add cucumbers, tomatoes, onion and olives to the bowl. Crumbled feta cheese over the top. Mix in dressing prior to serving.