One of the quickest and most effective methods of tenderizing oxtail is to use a tenderizer tool. This tool works by breaking down the tough fibers in the meat, making it more tender after cooking.  It generally takes just a few minutes to get an oxtail ready for cooking using one of these tools. Additionally, you can also marinate your oxtail overnight in a mixture of oil and acid, such as lemon juice or vinegar, to help break down those tough fibers before cooking.  Finally, braising your oxtail slowly over low heat can help break down the connective tissues, resulting in a more tender piece of meat when done. However, this method does take longer, so if you're short on time, a tenderizer tool may be your best bet. In conclusion, using a tenderizer tool or marinating overnight are the quickest and easiest ways to quickly make oxtail more tender before cooking. If you have the time, braising over low heat can also help make the meat more succulent and flavorful.