They are related, but not identical animals. Here is a page that explains the difference: http://www.differencebetween.net/science/nature/difference-between-bull-and-ox/ An ox is larger and stronger than a bull, and was used in the past for pulling farm machinery - hence the 'simile as strong as an ox'. Bulls are smaller, and are kept mainly for breeding. We don't talk about oxen very much these days - they mainly come up in grammar textbooks about nouns that have irregular plurals. One ox - two oxen.