Prepare chiles. Snip off stem ends. Empty out as many seeds as possible, discard seeds. No worries about the seeds, you'll be straining the blended sauce later.
Place chiles in bowl, cover with boiling water and top with a plate. Let reconstitute for about 15 minutes.
Remove chiles from water to blender, reserving soaking liquid. Add garlic, oregano and salt. Add some soaking liquid, about ¼ cup to start. Blend until smooth and thick, adding more soaking liquid as needed, up to about ½ to ¾ cups total. I find ½ cup soaking liquid to be ideal. Taste for salt, adding another ½ teaspoon, if needed.
Pour sauce into fine mesh strainer over a bowl and press through, leaving a small amount of solids behind. Transfer sauce to jar or container and store in refrigerator, or freeze. Makes about 1 cup.