Cook macaroni according to package directions in salted boiling water. Drain and place in a large bowl.
While the macaroni is still hot, stir in the oil, vinegar, onion, and seasoned salt. Let cool.
When the macaroni is cool, stir in the mayonnaise, diced cheddar, peas, ham, and celery.
Refrigerate until ready to serve.
Notes
You can make the macaroni salad up to one day ahead. Cover and refrigerate until ready to serve. Give it a stir before serving. Any leftovers can be stored in the fridge for up to 5 days. I don't recommend freezing the salad as the freezing/thawing process can affect the texture. Want to change it up? Add any of these: