Pair this garlicky salad with Barbecued Chicken on Garlic Toast.
Make dressing: In a small bowl, whisk together oil, vinegar, mustard, garlic, and 2 tablespoons water. Stir in oregano, if using; season with salt and pepper.
Remove dark-green outer leaves from each head of romaine, leaving just the paler-green hearts. (Reserve outer leaves for another use.) Halve hearts lengthwise. To serve, drizzle romaine with dressing.