The O blood group is one of the four main blood types, which also include A, B, and AB. It is characterized by the absence of A and B antigens on the surface of red blood cells. Individuals with this blood type can donate blood to any other group, making them universal donors, but they can only receive blood from other O type donors. People with the O blood group are often considered to have a lower risk of certain diseases, but they may be more susceptible to others. This blood type is the most common worldwide, with a significant percentage of the population having it.