Rinse the tomatoes, cut into quarters, remove seeds and cut into small cubes. Rinse cucumber, cut in half lengthwise, remove seeds with a small spoon and cut into small cubes.
Peel onion and chop finely. Cut feta into small cubes. Cut olives in half.
Place the prepared ingredients in a bowl with olives and oregano. Whisk oil with vinegar and salt and pepper to taste and mix with the salad. Serve garnished with oregano.