To make vinaigrette, whisk together vinegar, mustard, and oregano in a bowl. Slowly drizzle in oil, a little bit at a time, whisking vigorously until dressing is emulsified. After about half of the oil has been incorporated you can drizzle it in faster, but start out very slow at the beginning to create a nice, stable emulsion. Season to taste with salt and pepper.To assemble, rinse and dry greens. Toss with a few tablespoons of dressing until evenly coated. Arrange in serving bowl, then top with melon balls, mozzarella balls and prosciutto. Drizzle with more vinaigrette as desired, and sprinkle with a little flake sea salt and freshly ground black pepper, to taste. Serve immediately.