O-negative blood means that a person’s blood type is O in the ABO blood group system and RH-negative in the RH blood group system. Because there are dozens of blood group systems in the human body, but the ones that are widely used in clinical practice are the ABO blood group system and the RH blood group system. the ABO blood group system divides human blood types into four types, including O, A, B, and AB blood types. In the RH blood group system, human blood types are divided into positive and negative. Those containing D antigen are RH positive and those not containing D antigen are RH negative, so O negative blood means that a person’s blood type is O blood in the ABO blood group system and negative in the RH blood group system.