Preheat the oven to 450ºF. Line a baking sheet with aluminum foil. Coat the bell peppers lightly with olive oil and place on the baking sheet. Bake, turning every 10 to 15 minutes, until the peppers are blistered all over, about 30 minutes total. Transfer the peppers to a bowl and cover with plastic wrap so they steam as they cool. Peel the peppers and remove the stems, seeds, and ribs; avoid the temptation to rinse the peppers, which would wash away flavor. You should have about 4 cups.
Heat the 2 tablespoons olive oil in a small skillet over high heat. Add the whole chilies, lower the heat to moderate, and cook, turning occasionally, until the chilies are softened, lightly browned, and blistered on all sides. Remove the skillet from the heat and let the chilies cool in the oil for several minutes. When cool enough to handle, peel the chilies and remove the stems and seeds (or leave some or all of the seeds if you prefer a spicy sauce). Chop the chilies finely, then use the side of your knife to mash them to a paste.
Add more oil to the skillet if needed to make 2 tablespoons. Reheat the oil over moderate heat and add the garlic. Cook until lightly browned, about 30 seconds, then add the oregano and sauté briefly to release its fragrance. Add the tomato puree and salt, bring to a simmer, and simmer for about 5 minutes to thicken slightly.
Combine the red peppers, serrano chilies, and tomato sauce in a blender and puree until smooth. Add the vinegar and the black pepper to taste and puree again. Taste and adjust the seasoning. Refrigerate for up to 5 days or freeze for up to 4 months.