Cut the vegetables; nothing in a horiatiki salad should be bigger than bite-sized. If the onion is a bit too sharp for your taste, you can soak the slices in ice-water for up to an hour, to remove some of the heat (or just use less).
Strip oregano leaves from the stems, and run a knife through them to chop them a bit smaller, and to release their flavors (you’ll smell this – mmm, fresh!). Put the leaves in a bowl, and add salt if using, and then the olive oil and vinegar. Whisk well to combine (nothing fancy here, a fork works) then add all the chopped vegetables and olives. Toss lightly.
Top with crumbled feta cheese and serve at room temperature or slightly chilled. Add a squeeze of fresh lemon if you’d like a bit more zip.