Whisk together the red wine vinegar, mustard, oregano and garlic in a large mixing bowl. Slowly stream in the olive oil, continuing to whisk until emulsified. Season with salt and pepper. Add the salami, mozzarella, tomatoes and basil to the bowl and toss to coat with the dressing. Transfer to a serving bowl. If serving right away, toss with basil; otherwise refrigerate for up to 4 hours and toss with basil right before serving.