In Mexico, just about any leftover transforms into a taco filling.
Put potato and ¼ cup water into a large microwave-safe bowl. Sprinkle generously with salt. Cover with plastic wrap, poke a couple of holes in the top and microwave on high (100 percent power) until tender, but not mushy, about 4 minutes. Use a slotted spoon to move potatoes onto a cutting board (save the cooking liquid). Let potatoes cool.
To the bowl with the potato-cooking water, add the vinegar, oregano, chipotle chiles and onion. Mix well, then taste and season with salt, usually about 1 teaspoon.
Use a fork to break the cooled potatoes into ½-inch pieces, then scoop them into a large bowl. Add the chicken, then drizzle on the dressing and toss everything together. (If there is time, refrigerate the mixture for about half an hour to blend flavors.)
Just before serving, add the lettuce and avocado to the bowl. Drizzle with the oil and toss to combine everything. Serve with warm tortillas for making soft tacos.