There's an extremely rare blood type Oh, aka the Bombay Phenotype! Just how rare is it?

People with blood type A have the A antigen. People with type B have the B antigen, while people with AB have both. People with type O blood have the H antigen, which is found on virtually all red blood cells and is the building block for the production of the antigens within the ABO blood group.

There are some who are H antigen deficient. This is known as h/h, Oh, or, most commonly, the Bombay Phenotype. It's very rare, affecting 1/10,000 people in India, where it was first discovered in 1952, and 1/1,000,000 people in Europe.

This phenotype can be the cause of confusion in paternity tests because these people are not only missing the H antigen, but because it is the basis of the A and B antigens, people with the Bombay Phenotype are missing these antigens too. For this reason, blood type Oh can be confused with blood type O, which is missing the A and B antigens but does contain the H antigen.

There are no ill effects of the Bombay Phenotype, but if someone with this blood type needs a blood transfusion, they must get one from someone else with the same blood type. Since it is missing the H antigen, they cannot accept type O blood. Because of this and the rarity of the blood type, most transfusions come from family members. These people can, however, donate blood to any member of the ABO blood group.

(Source)