Wash potatoes thoroughly. Place them in a large pot, cover with cold water, and add a generous pinch of salt. Bring to a boil and cook until fork-tender, about 15-20 minutes depending on size. Drain well and let cool slightly.

While potatoes are cooking, prepare the dressing. In a small bowl, whisk together olive oil, lemon juice, Dijon mustard, minced garlic, oregano, salt, and pepper until well combined.

Once potatoes are cool enough to handle, cut them into bite-sized cubes. Place the cubed potatoes in a large mixing bowl.

Add the chopped parsley, dill, sliced scallions, diced celery, diced red bell pepper, and halved kalamata olives to the bowl with the potatoes.

Pour the prepared lemon dressing over the potato mixture. Gently toss everything together until all ingredients are evenly coated with the dressing. Be careful not to overmix, which can mash the potatoes.

Taste and adjust seasoning if necessary, adding more salt, pepper, or lemon juice as desired. For best flavor, cover and refrigerate for at least 30 minutes before serving to allow the flavors to meld.