Oxen are bovines specialized as draft animals. Both oxen and bulls are domesticated bovines, and oxen are always male animals. However "bull" is a general name for any male bovine, whereas an ox is such an animals which has been specially trained and physically prepared for hard labor pulling and carrying loads.
Part of this physical preparation is castration. A bull that is to become an ox has its testicles removed when it is young in order to make it more docile. Intact bulls are less readily used as draft animals as they are not as patient and can become aggressive when handled. Female animals, on the other hand, are more docile, but they are not as suitable for draft work as males because they are usually smaller and weaker. In addition, it is more useful to have female animals producing milk or gaining weight for beef, activities which would be at odds with the long hours and heavy labor of draft work. Rarely, when no male animal is available, cows have been used as oxen instead of castrated bulls, but that is not the usual practice.
Another part of the physical preparation of a bull into an ox begins before the animal is born. Artificial selection, also known as animal breeding, produces stronger and larger animals down through the generations. There is no such thing as a single ox breed, but dairy cattle are known to be more gentle and energetic than beef cattle breeds, and beef breeds are larger and more muscular than dairy. A farmer desiring an ox that is both exceptionally gentle and exceptionally large may crossbreed a dairy cow with a bull from a beef breed to produce such a combination of useful traits.