In a medium bowl, whisk together the red wine vinegar, lemon juice, Dijon mustard, oregano, kosher salt and black pepper.
Gradually whisk in the olive oil, adding 1 tablespoon at a time and whisking until it incorporates, until an emulsion forms. Serve immediately. Store refrigerated and bring to room temperature before serving (keeps at least 2 weeks). The dressing may separate after sitting which is okay; simply whisk again before serving.