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.
If you don't have seedless cucumbers, you can use regular cucumbers. Just peel them if the skin feels thick, and cut them in half lengthwise and scrape out the seeds before dicing them. Baby or Persian cucumbers are another option; use 3 or 4 of those.
You can use pre-crumbled feta instead of cubing feta
If you just want to use fresh oregano, use about 1 1/2 tablespoons. If you want to use just dried, use 1 1/2 teaspoons