Why do scuba divers carry oxygen cylinders?

Scuba divers do not carry oxygen cylinders, they carry air cylinders. Air cylinders contain compressed air, which is mostly nitrogen (around 78%) with a small amount of oxygen (around 21%) and other gases. This compressed air is used by the diver to breathe underwater. Oxygen cylinders are used by some divers in specific situations, such as when diving in very deep water or in emergencies, but they are not typically used by recreational scuba divers.