Cook the rotini in salted water until just al dente. Drain and rinse under cold water to stop cooking. Let cool completely.
While pasta cools, chop vegetables into bite-sized pieces. Slice onions thin; cut mozzarella into small cubes if needed.
In a large bowl, combine cooled pasta, vegetables, olives, and cheese. Toss gently.
Pour Italian dressing gradually over the mixture, tossing to coat. Add oregano, garlic powder, salt, and pepper. Mix well and adjust seasoning to taste.
Cover and refrigerate for at least 1-2 hours. Before serving, toss again and add a splash of dressing if needed.