Oxtails are a tougher cut of beef; they are the tail of a cow, which means that it needs longer cooking times to break down the muscle fibers and get nice and tender. That’s why we typically cook it by braising it. In a standard crockpot, oxtail generally takes around 6 hours to get fork-tender. Luckily, the Instant Pot significantly cuts down the time and takes just 90 minutes to cook the oxtails to perfection.