Cook the Pasta: Bring a large pot of salted water to a rolling boil. Add the rotini pasta and cook according to package instructions, usually about 8-10 minutes. Drain and rinse under cold water.

Combine Ingredients: In a large mixing bowl, combine the cooled pasta, sliced pepperoni, diced mozzarella, halved cherry tomatoes, sliced olives, diced bell pepper, and chopped onion. Mix gently.

Dress the Salad: Drizzle the Italian dressing over the pasta mixture, then toss until everything is well coated.

Season the Salad: Sprinkle oregano, salt, and pepper over the salad, folding them in gently.

Chill: Cover with plastic wrap and refrigerate for at least one hour.

Toss Before Serving: After chilling, gently toss the salad and adjust seasoning if needed.

Serve Cold: Spoon the salad into serving bowls or platters and enjoy.