O−, or O negative, is a blood type characterized by the absence of A and B antigens on the surface of red blood cells. It is part of the ABO blood group system and is considered a universal donor type, meaning that individuals with O− blood can donate to any other blood type in emergency situations. This blood type is relatively rare, found in about 7% of the population. People with O− blood can only receive blood from other O− donors, making it crucial for blood banks to maintain an adequate supply for those in need, especially during emergencies or surgeries.