Bring a large pot of lightly salted water to a boil. Add cavatappi and cook, stirring occasionally, until tender yet firm to the bite, 9 to 10 minutes. Drain and run under cold water until no longer hot. Drain again and transfer to a big bowl.
Add mayonnaise, tuna, olives, celery, onion, salt, and pepper to the pasta; toss until fully combined. Serve immediately or chill in the refrigerator until ready to serve.