Add the olive oil to the One Pot (or any pressure cooker, or a Dutch oven) and heat over medium heat until hot. Season the oxtails with the salt and pepper. Add half of the oxtails to the One Pot and cook until well browned on both sides, about 3 to 4 minutes per side. Set the seared oxtails on a platter once browned and repeat with the remaining oxtails.
Once all the oxtails are browned, add the onions, carrots and celery to the pan. Saute the vegetables until softened, about 3 to 4 minutes. Add the leeks, garlic, green onions and ginger to the pan and saute for 1 minute, stirring often. Add the tomato paste and stir well to incorporate. Sprinkle the flour over the vegetables and stir to combine. Cook, stirring, for 3 minutes. Add the beer to the One Pot and increase the heat to medium-high. Bring the beer to a boil and cook for 5 minutes.
Add the stock and allspice, stir, and return the oxtails to the pan. Return the pan to a boil and place the lid over the pan. Once a steady stream of steam is emitted from the pan, lower the heat to a simmer, and cook the oxtails until tender, about 1 hour in a pressure cooker or 2 hours in a Dutch oven. Remove the lid, and stir the chopped parsley into the pan. Serve immediately.