Place pressure cooker, uncovered, over high heat.
Place the oil, salt, and the oxtail and shank pieces in a large metal bowland toss to coat.
Once the pressure cooker is hot, transfer half the pieces to the cooker and sear on both sides until golden brown. Remove the pieces and repeat with the second half. Wipe out any excess fat from the bottom of the pot with a paper towel held with tongs.
Return all of the meat to the pressure cooker and add the remaining ingredients. Be careful not to fill above the cooker's "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 to low, so that you barely hear hissing from the pot. Cook for 50 minutes.
Release pressure using your cooker's release device (read the manual), or cool the cooker by running cold water over the lid for 5 minutes. Carefully open the lid. Set a cheesecloth-lined colander into a large pot and strain. Taste and season with additional salt, if desired. A squeeze of lemon or splash of sherry makes a delicious addition as well.