Toast the chiles in a pan over medium-high heat, turning occasionally, until pliable and fragrant, 3 to 4 minutes. Add the chicken stock and bring to a boil. Cover, remove from the heat, and allow to stand until the chiles are very soft, 20 to 25 minutes. 

Combine the chiles, their soaking liquid, chopped onion, garlic, salt and oregano in a blender and puree until smooth. Strain through a fine mesh sieve to discard any pieces of skin or seeds. Discard solids.

Chile sauce may be used in soups, stews and marinades. Any remaining chile sauce freezes well.