In a small bowl, combine olive oil, wine vinegar, oregano, parsley, and onion. Mix and set aside.
Add potatoes to a large pot of boiling, lightly salted water. Boil for about 20 minutes, or until potatoes are tender. Drain water and let cool.
When cooled, peel and dice. Place in a large bowl, add dressing and season with salt and pepper to taste. Toss to coat, cover bowl and refrigerate for several hours or overnight.
