Sauté celery in butter in a small saucepan until tender. Stir in oregano, salt, garlic powder, and pepper.
Wash potatoes; cut each into 1/2-inch slices, slicing down to, but not through, bottom peel. Place a slice of onion between each slice of potato.
Arrange each potato on a square of heavy-duty aluminum foil and drizzle each with about 2 tablespoons of butter mixture. Fold edges of foil over and wrap each potato securely. Grill for one hour or until done.