In a large pot of boiling water, cook the ditalini until al dente as per package directions. Drain the pasta and place in a large tupperware.
Toss the pasta with the olive oil. Chill in the refrigerator at least 15 minutes.
Finely dice the celery, onion, and tomato. Place the onion only in a small bowl and cover with cold water. Let sit 5 minutes and then drain. (This will take some of the sharpness out of the raw onion).
In a small bowl, combine the mayo, dry mustard, sugar, vinegar, sour cream, and 1/2 tsp salt.
Take the pasta out of the refrigerator. Add the vegetables, sauce, and parsley. Stir to combine. Season with salt and pepper to taste. Serve immediately or chill until needed.
This macaroni salad has a little tang to it from the vinegar. If you are not a fan of a vinegar taste, reduce or omit the vinegar at first and then add a little at a time to taste preference.