"OC" stands for oleoresin capsicum, and is the primary ingredient in pepper spray. It is extremely hot and, when inhaled, inflames the nerves with which it comes into contact. It causes swelling in the eyes and inflames the mucous membranes in the respiratory tract, which makes breathing difficult.
Oleoresin capsicum, in the initial form of capsaicin, is a naturally occurring ingredient in peppers. It was first used as pepper spray by the FBI and postal employees in 1973 for the incapacitation of violent suspects, dogs and other animals. In rare cases, the use of pepper spray has caused death when a pre-existing respiratory condition was present in the victim or a positional restraint was used in conjunction with the spray.