Wikipedia says, “Oxtail soup is made with beef tails. The use of the word “ox” in this context is a legacy of nomenclature; no specialized stock of beef animals are used.”
“Though translated literally as a soup, this version of the dish is somewhere between a soup and a stew. One of the defining characteristics of oxtail soup is that it contains a large mass of solid ingredients, rather than ingredients that have been diced or shredded as is the norm with Chinese soup. Chunks of oxtail, potato, carrots, cabbage, tomato, and mushrooms are mixed in water, and salted to taste. The mixture should be heated at a slow boil, to allow time for the ingredients to release their flavors. In particular, the tomatoes and potatoes should largely disintegrate into the water, giving the broth a reddish-orange coloration and thickening it. As to be expected, oils from the oxtail lend most of the flavor. The soup is served with all of the ingredients.”
… in Chinatown … A pot of oxtail stew burbles on the stovetop, fills the room with a deep, rich cloud of stewiness, fills the room like a fog bank rolling over the bay.