Place potatoes into a large pot. Boil potatoes in generously salted water until fork tender, then drain.
Transfer ⅓ of the potatoes to a large mixing bowl and mash until creamy.
Once remaining potatoes are cool enough to handle, quarter and add to the bowl of mashed potatoes.
Add boiled eggs, pickles, shallots, chives, oregano, salt, and pepper to the bowl and gently fold together.
In a small bowl, whisk together all the ingredients and pour into the potato mixture.
Fold dressing into the potato mixture until completely combined.
Cover and place mixture in the refrigerator for at least 2 hours and up to 1 day. Remove from refrigerator, adjust seasoning and serve.