Add the sliced cucumber and red onion to a large bowl.
In a small bowl, whisk together the sour cream, mayonnaise, lemon juice, white vinegar, fresh dill, and dried oregano until smooth. Season with salt and pepper to taste.
Pour the dressing over the cucumber and onion mixture and toss gently until evenly coated.
Cover and refrigerate the salad for at least 20 minutes to allow the flavors to blend.