Preheat the oven to 325°. Heat the oil in a large flameproof casserole. Season the oxtails with salt and pepper and cook over moderately high heat, turning often, until browned all over, about 20 minutes. Transfer the oxtails to a platter.
Pour off any fat from the casserole and add the wine and beef stock. Bring to a boil over high heat, stirring to incorporate any browned bits from the bottom of the pan. Return the oxtails to the casserole and add the chopped onions, carrots and celery and the chocolate, tomato paste and bay leaves. Cover and return to a simmer, then transfer to the oven and bake for about 4 hours, turning the oxtails often, until the meat is meltingly tender; add about 1/2 cup of water after 3 hours if the liquid has evaporated.
Meanwhile, in a small bowl, soak the prunes in 1 cup of warm water until plumped, about 20 minutes; drain.
Transfer the oxtails to a large platter and cover to keep warm. Strain the cooking liquid through a sieve and skim off the fat. Add water, if necessary, to make 2 1/2 cups of liquid. Return the liquid to the casserole and add the prunes.
In a large skillet, cook the pancetta over moderately high heat, stirring often, until browned, about 5 minutes. Pour off any fat, add the pearl onions and cook, stirring often, until golden, about 7 minutes. With a slotted spoon, transfer the pancetta and onions to the casserole and simmer until the onions are tender, about 5 minutes. Pour the sauce over the oxtails and serve with the pappardelle