Whisk ¼ cup extra-virgin olive oil, 3 tablespoons red wine vinegar, 2 teaspoons dried oregano, 1 minced garlic clove, 1 teaspoon kosher salt, ½ teaspoon black pepper, and ½ teaspoon red pepper flakes together in a large bowl. Stir in ½ medium thinly sliced red onion and let sit for at least 10 minutes.
Stir in 1 (about 15-ounce) can each drained and rinsed cannellini beans and chickpeas, 12 ounces halved cherry tomatoes, 8 ounces drained fresh mozzarella cheese pearls, 4 ounces diced provolone cheese, 3 ounces salami sliced into thin strips, 4 coarsely chopped pepperoncini peppers, and ½ cup each coarsely chopped fresh basil and parsley leaves if using. Taste and season with more kosher salt and black pepper as needed.
