Peel the cucumbers; cut them in half and use a spoon to scoop out the seeds, then chop them into bite-sized chunks (you can skip peeling and seeding for English cucumbers). Coarsely chop the tomatoes. Dice the onion. Chop the oregano.
In a large bowl, mix the cucumbers, tomatoes, onion and oregano with the Kalamata olives. Mix in the the olive oil, lemon, fresh ground pepper, and kosher salt. Add a splash or two of white wine vinegar. Taste, and add more salt or pepper if desired.
Crumble or cut feta cheese into chunks, then add it to the salad. Let stand for 5 minutes to meld flavors, then serve.