Preheat a grill to medium-high. In a pot, combine the potatoes and enough salted water to cover by 1 inch; cover and bring to a boil. Lower the heat and simmer until fork-tender, about 5 minutes. Drain in a colander, rinse with cold water and shake to remove any excess water.
In a large bowl, stir together the mayonnaise, horseradish and parsley. Stir in the apple, onion and celery; season with salt and pepper. Stir in the potatoes and drizzle with 1 tablespoon olive oil; gently stir to combine, then refrigerate.
Drizzle the pork chops with olive oil and season with salt and pepper. Grill, turning once, for about 12 minutes for medium. Serve with the potato salad.