ABO blood group system
The use of O as a number can lead to confusion as in the ABO blood group system. Blood can either contain antigen A (type A), antigen B (type B), both (type AB) or none (type O). Since the "O" signifies the lack of antigens, it could be more meaningful to English-speakers for it to represent the number "oh" (zero). However, "blood type O" is properly written with a letter O and not with a number 0.