O-type refers to a classification of blood type characterized by the absence of A and B antigens on the surface of red blood cells. Individuals with O-type blood can donate to any other blood type, making them universal donors. However, they can only receive blood from other O-type individuals. In the context of the ABO blood group system, O-type blood is further divided into O-positive and O-negative, depending on the presence of the Rh factor. O-negative blood is particularly valuable in emergency situations, as it can be given to patients of any blood type without risk of an adverse reaction.