On the Instant Pot press on SAUTE twice to choose the MEDIUM setting, and heat the oil. The screen will show the HOT message. Add the oxtails to sear. Season with salt and pepper, then start moving them around until browned from all sides. If you feel like your pot is overcrowded, work in batches with more oil.
When that is done, switch off the saute setting. Add the wine if using, and broth and deglaze with it.
Add the carrot, celery, onion, garlic, and tomato paste. Give everything a good stir.
Add a bay leaf, and some fresh or dry thyme.
Secure the lid and set the vent to SEALING. Select the PRESSURE COOK/MANUAL setting and set the cooking time for 45 minutes at high pressure. The Instant Pot will take around 10-15 minutes to come to pressure and start cooking. When the cooking program ends, allow the steam to release naturally.
Carefully remove the lid, and the oxtails are ready to be served.
Skim fat from the liquid if using, and thicken with cornstarch slurry.