Prepare the vegetables: Cut the tomatoes into wedges, slice the cucumber into half-moons, and thinly slice the red onion and green bell pepper.
In a large salad bowl, combine the tomatoes, cucumber, red onion, green bell pepper, and Kalamata olives.
Place the blocks of feta cheese on top of the salad.
Drizzle the olive oil and red wine vinegar (or lemon juice) over the salad.
Sprinkle the oregano, salt, and freshly cracked black pepper on top.
Gently toss the ingredients together, making sure to keep the vegetables intact and not to break the feta cheese blocks.
Serve immediately, or store the salad without the dressing for later use.