Heat a dry skillet or comal over medium heat. Toast the árbol peppers 30-60 seconds per side, until fragrant. Do not burn.
Transfer toasted chilies to a saucepan. Add the onion, garlic, vinegar, water, oregano, and salt. Bring to a boil, then reduce heat and simmer 10 minutes, until the chilies are softened.
Carefully transfer mixture to a blender. Add lime juice and blend until smooth. Add a splash more vinegar or water if needed for consistency.
Adjust to taste with salt, vinegar, or lime as desired.
