O型, or blood type O, is one of the four main blood types in the ABO blood group system. It is characterized by the absence of A and B antigens on the surface of red blood cells. People with O型 blood can donate to any other blood type, making them universal donors, but they can only receive blood from other O型 donors. In terms of genetics, O型 blood is determined by the presence of two O alleles, which are recessive. This means that a person must inherit an O allele from both parents to have O型 blood. The distribution of O型 blood varies among different populations, with higher frequencies found in certain ethnic groups.