Cut the red onion in half and then into thin slices. Place them in a small bowl and cover with water and a tablespoon of vinegar.
Cut the peeled cucumber in half lengthwise, then slice into thick halves.
Cut the tomatoes in half. Then cut them again to make quarters and finally into wedges or large chunks.
Slice the green bell pepper into rings.
Place everything in a large salad platter/dish/bowl.
Add as many kalamata olives (and capers - optional) as you like.
Drain well the onions and place them on top of the vegetables.
Top with the feta cheese block or/and big chunks.
Pour the olive oil all over and sprinkle a generous pinch of oregano. If the oregano isn't too strong, add some more. Don't add too much because the salad will be bitter.
If the ingredients are in season and the olive oil is extra top-quality, the vinegar isn't necessary. Taste some and add to taste.
Serve and mix the ingredients at the last minute.