The species is cattle (unfortunately, like "people" there is no proper singular).
Oxen are fully mature (over about 4 years old) castrated male cattle, although the term is sometimes used more generally for any draught cattle, as in "ox-cart".
Younger castrated males are steers or bullocks, and of course entire males of any age are bulls. Oxen are taller and lighter-built than bulls, with muscular shoulders instead of a "bull-neck". Steers are much the same shape as females.
Young females are called heifers, and after their first calf they are cows.
The meat of all of them is beef, except calves which are of course veal. Related species would also be beef and would have oxen, heifers etc: water buffalo, African buffalo, yak, kouprey, mithun, bison (called buffalo by Americans) etc etc.
Oxen are extremely rare in the UK nowadays. This is because they can now only be used for beef, and beef is always killed young. In places where cattle are used for draught, or are reared less intensively, oxen are much more common.
Commercial intensive farmers in Britain (that is, nearly all of them) usually kill at about 20 months. Currently the law in the UK forbids eating animals killed over 30 months -- this was originally to protect people from BSE, but is now no longer needed. It is an ongoing scandal that it has not been changed. Our own beef is killed at 30 months, and even that makes supermarket beef look like veal.
We did have an ox until recently. He had to go, so was incinerated under the government scheme -- a shocking waste of hundreds of kg of perfect beef. I could get quite upset about it. Most over-age animals are of course old bulls and cows past breeding or milking, but perfect for pies, dog food etc -- those things are now made from prime young beef.