In a container combine the olive oil, vinegar, 1 teaspoon dried oregano, mustard, salt, garlic, pepper, and red pepper flakes (if using). Shake well to combine.
If you are using a seedless cucumber (which of course are never fully seedless), slice the cucumber in half lengthwise, and scrape out the seeds with a teaspoon. Slice the cucumber. If you are using small person cucumbers, simply slice them.
In a large mixing bowl, combine the lettuce, red onion, tomatoes, and cucumber. Shake the dressing once more to combine, and pour over about half of the dressing. Toss the salad. Transfer to a shallow serving bowl. Place the slices of feta on top, and drizzle over a bit more of the dressing. Sprinkle over a bit of dried oregano, and the fresh oregano leaves, if using.