The Chinese method of red cooking, or red braising, involves cooking meat with a scattering of sugar. The sugar, which caramelises on the surface of the meat, pumps up the naturally sweet flavour that comes with searing the surface of any protein. Once the browned bits of the oxtail simmer for a couple hours, the entire broth takes on a rich, sweet flavour.
Chinese mustard greens are also known as mustard cabbage and sow cabbage. They'll add a little heat to the rich caramelised meat, but if you can't find them online you can use kale instead.