A cow and an ox are both the same species of animal, but may be different breeds. Primarily, cows are kept for food and reproduction, while oxen are trained for use as work animals. There are many other differences between them.
Cows are female cattle that are over 2.5 years of age or have had at least one calf. Before meeting these requirements, they are called "heifers." These animals are kept for their milk and meat, but may also be used as work animals. However, their primary purpose is for food. Cows tend to be smaller than working oxen. The most popular breed of dairy cow in the United States is the black and white Holstein.
Oxen are fully-grown, castrated male cattle that are 4 years of age. Before this age, they are called "steers." The ox is usually from the Box indicus or the Bos taurus genus. Oxen are selected for their size, strength, personality and ability to learn. An ox is usually the same size as a full-grown bull, but is taller and leaner in the chest. The Holstein is also a popular choice for oxen.Learn more about Barnyard Mammals