In a large bowl, combine the cucumber, tomatoes, bell pepper, and red onion.
Add the olives and capers (if using).
In a small jar, whisk together olive oil, red wine vinegar, lemon juice, garlic, oregano, salt, and pepper until emulsified.
Pour the dressing over the vegetables and toss gently to coat.
Top with the feta (leave in slabs for a traditional look or crumble), sprinkle with a pinch of oregano, and serve immediately or chill 10–15 minutes.