Put chiles and 5 cups water into a medium pot, partially cover pot, and bring to a boil over high heat. Reduce heat to medium-low and gently simmer until chiles are soft, about 30 minutes.
Meanwhile, heat oil in a small cast-iron or other heavy skillet over medium-low heat. Add garlic and cook, turning cloves occasionally, until soft and golden brown all over, about 15 minutes.
Transfer chiles with a slotted spoon to a clean surface and discard all but 1 cup of the cooking liquid. Pull off and discard chile stems, then put chiles into a blender. Add reserved cooking liquid and garlic and puree until sauce is smooth. Add oregano, crushing it gently between your fingers, and season to taste with salt. Transfer sauce to a bowl. Serve as a sauce with red chile tamales (see recipe, facing page) or Sonoran enchiladas, if you like, or scrambled eggs wrapped in warm tortillas. Author Guerrero also sometimes tosses shrimp sauteed with garlic with the sauce. The sauce keeps covered in the refrigerator for up to 2 weeks.