Heat the oil in a medium saucepan over medium high heat and quickly toast the chiles for 10-15 seconds. They will brighten in color. Quickly remove from the oil so they don't burn and transfer them to a food processor or blender.
Add the tomatoes, tomatillos, vinegar, garlic, salt, oregano and a 1/4 cup of water to the blender. Process until smooth. Season with salt as needed.
Pour mixture into saucepan and bring to a boil, then reduce heat to low and simmer for 15 minutes. If the consistency is too thin for your liking, continue simmering until it reduces and reaches your desired consistency.
Allow salsa to cool before using. Serve either warm or chilled.