Puree chilies in blender. Transfer to large bowl. Heat 1 teaspoon oil in heavy large skillet over high heat. Add tomatillos and cook until brown on all sides, about 7 minutes. Transfer to work surface. Add 2 teaspoons oil to skillet. Add onion and saute until tender, about 4 minutes. Add onion to chilies. Chop tomatillos; add to chili-onion mixture. Mix in cilantro, vinegar and oregano. Season with salt and pepper. (Can be made 2 days ahead. Cover; chill. Bring to room temperature.) *Chipotle chilies and tomatillos are available at Latin American markets, specialty food stores and some supermarkets. Makes 1- half cups.