At birth a calf is male or female. A male calf is called a bull, a female is a heifer. Now we get changes of names as they age. At 3 years old, a female heifer becomes a cow. At 6 to 9 months if a bull calf is castrated, he is now a steer. A steer at 4 years old and with training has become an ox! If a male calf is not castrated he stays a bull.
Most any breed of cattle can be used for oxen. They must have horns. The horns keep the yoke on their necks. Without the horns they could put their heads down and the yoke would slip off their necks. Mostly the dairy breed of cattle are used, because they are willing to work after they have been castrated. Only the steers are used, cows are not used as oxen, because they will not grow as big as an ox or be as strong.