Bring potatoes to boil in lightly salted water in large saucepot. Reduce heat and simmer 10 minutes or until fork-tender. Drain well. Cool slightly.
Mix oil, ketchup, vinegar, mustard, black pepper and salt in large bowl with wire whisk until well blended. Add potatoes, celery, bell pepper and onion; toss gently to coat.
Serve immediately or refrigerate until ready to serve.