Place the tomatillos, chiles, and water in a medium saucepan and bring to a boil over high heat. Cover the pot and reduce the heat to a steady simmer. Cook, stirring occasionally, until the tomatillos are very soft and beginning to burst, usually 10 to 20 minutes, depending on their size. Remove from the heat.

Using a slotted spoon, transfer the tomatillos and chiles to a blender along with some of the water remaining in the pot. Add the onion, garlic, and cilantro and blend until nearly smooth, adding more of the water from the saucepan as needed to form a saucelike consistency.

Transfer the remaining water to a bowl and set aside. Rinse and dry the saucepan, return it to the stovetop over medium high heat, and add the olive oil. When hot, pour the sauce into the saucepan and add the oregano and salt to taste. Cook the sauce for 5 to 10 minutes, just until flavors come together, adding a bit more of the reserved cooking water should the sauce become too thick. Set sauce aside to cool and use as desired.