Place pressure cooker over high heat. Oil and salt the oxtail and shank pieces then sear in batches. Add remaining ingredients and cover with water, being careful not to fill above the cookers "maximum fill" line. (If your pressure cooker does not have a water line, fill the pot 2/3 full). Bring to a boil and skim off any foam that gathers at the surface. Cover and lock lid. Once pressure builds up inside the cooker, reduce the heat so that you barely hear hissing from the pot. Cook for 50 minutes.
Release pressure using your cookers release device (read that manual) or cool the cooker by running cold water over the lid for 5 minutes. Carefully opening the lid and strain squeezing the solids before feeding to the compost pile, or the dogs. Strain through a fine sieve or several layers of cheesecloth. Season and serve or use as a base for other soup recipes.