In a large mixing bowl, combine diced cucumber, halved grape tomatoes, diced bell pepper, sliced red onion, Kalamata olives, and crumbled feta cheese.

In a separate small bowl, whisk together red apple vinegar, lemon juice, Dijon mustard, minced garlic, oregano, salt, and black pepper. Gradually add olive oil while whisking until emulsified.

Drizzle the dressing over the salad mixture and gently stir to combine until all ingredients are evenly coated.