Cook pasta in boiling salted water according to package directions. Drain. Rinse pasta under cold running water; drain. Stir in oil until coated. Cover and chill 30 minutes. Meanwhile, combine mayonnaise and next 7 ingredients in a large bowl, stirring until blended and smooth. Add pasta, stirring until pasta is creamy and coated. Stir in onion and remaining 2 ingredients. Serve immediately, or cover and chill 1 hour.