My dictionary says that normally "oxen" means male cattle (usually castrated) used for carrying or hauling, or being fattened as food. But it adds that in the zoological sense it covers "any beast of the bovine family of ruminants, including the domestic European species, the 'wild oxen' preserved in certain parks in Britain, the buffalo, bison, gaur, yak, musk-ox etc". So everyone's right, and there's no need to know an ox from a bullock!