Preheat your oven to 400°F.
Add the chopped potatoes, olive oil, and salt and pepper to a large mixing bowl. Toss to coat evenly. Spread the prepared potatoes in a single layer onto a large baking sheet.
Roast the potatoes for 45 minutes to 1 hour, turning halfway through the baking time. Potatoes are done when they are golden brown and fork tender. Let the potatoes cool completely.
Once the potatoes have cooled, mix together the Greek yogurt and onion soup mix in a large mixing bowl.
Add in the cooled potatoes and chopped celery; mix well. Season with salt and pepper to taste. Top with chopped green onions.
Cover and refrigerate until ready to serve.