For the oxtail braise: Heat the olive oil in an 8- to 10-quart stovetop pressure cooker. Meanwhile, sprinkle the oxtails generously with salt and pepper and add to the oil. Sear the oxtails on both sides, about 5 minutes per side. Add the onions, carrots, celery, guajillo chiles, garlic, tarragon, thyme, parsley, black peppercorns, avocado leaves, bone broth, Mexican cola and demi-glace. Lock on the pressure cooker lid and pressure cook at high pressure for 30 minutes. Carefully quick-release the pressure, then unlock the lid and open.