Cut the cucumber in half lengthwise, scrape out the seeds (see Note), then sliced again lengthwise through the middle, and slice into 1/4-inch slices. Cut the tomatoes into one each pieces. Cut the onion into 1/2-inch dice (it will fall apart, that’s fine).
In a large bowl mix together the olive oil, lemon juice, vinegar, fresh oregano, dried oregano, salt and pepper. Add the cucumber, tomatoes, onion, and olives to the bowl and toss to coat well. Top with the feta cheese and serve.
Even “seedless” cucumbers have some seeds in them – scrape them out with a teaspoon, or if they don’t bother you, skip this step.