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 2/3 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.
Notes
You do not have to sprinkle the salad with some fresh oregano leaves, and I would not go out of your way to buy them if they aren’t already in the fridge or in the garden, unless you are making Greek salad for a special occasion. It’s a lovely touch, but this is one of the only salads where I think dried herbs (in this case oregano) add that classic flavor to the dressing. So, in short, if you can use a bit of both fresh and dried (not too much), that is the optimal Greek Salad oregano experience.