The term phenotype refers to the observable characteristics of an organism, such as hair color, eye color, and blood type.

Blood type is determined by the presence or absence of certain antigens on the surface of red blood cells. The ABO blood group system is the most common blood typing system, and it has four main blood types:

* A: Individuals with blood type A have the A antigen on their red blood cells.

* B: Individuals with blood type B have the B antigen on their red blood cells.

* AB: Individuals with blood type AB have both A and B antigens on their red blood cells.

* O: Individuals with blood type O have neither A nor B antigens on their red blood cells.

So, "Phenotype O" actually refers to the blood type O, which is characterized by the absence of both A and B antigens on the surface of red blood cells.