Dice all vegetables into small, uniform pieces for consistent texture.
Rinse and drain the black beans and cannellini beans thoroughly.
In a large bowl, combine the beans, cucumbers, tomatoes, bell peppers, red onion, olives, parsley, and feta.
In a small bowl, whisk together olive oil, lemon juice, garlic, oregano, salt, and pepper.
Pour the dressing over the salad and gently toss until everything is evenly coated.
Cover and refrigerate for at least 30 minutes to allow flavors to meld.
Stir gently before serving. Serve chilled as a dip, salad, or topping.