In a large pot, bring salted water to a boil. Add the rotini pasta and cook according to package instructions until al dente. Drain and rinse under cold water to stop the cooking process.
While the pasta is cooking, chop the cherry tomatoes, cucumber, bell peppers, and red onion. Slice the black olives and mozzarella balls.
In a large mixing bowl, combine the cooked pasta, cherry tomatoes, cucumber, olives, bell peppers, red onion, and mozzarella balls.
Pour the Italian dressing over the pasta and veggies. Sprinkle dried oregano, salt, and pepper. Toss everything together until evenly coated.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. Just before serving, give it another toss and garnish with fresh basil leaves.