If my blood type is O positive, what kind of blood types did my parents have?

Your parents could be either A, B, or O, and at least one of them would have to be Rh positive. The one certainty is that neither of them could be AB.

The genes that code for everything are present in pairs and so too are the surface antigens that determine

To explain: A person who is blood type A could be genetically either AA or AO (receiving a gene that codes for A from one parent and a gene that codes for either A or O from another parent). A person who is blood type B could be genetically either BB or BO. A person who is blood type AB has to be genetically AB.

A person who is blood type O has to be genetically OO (receiving a gene that codes for neither A nor B from one parent and a gene that codes for neither A or B from another parent). The possible blood types of the parents include OO, AO, and BO.

A person who is blood subtype Rh positive could be genetically either DD or Dd (receiving a gene that codes for D from one parent and a gene that codes for either D or the absence of D [d] from another parent).

As can be seen, a person who is O positive could receive the gene for absence of A and B from parents who are genetically AO, BO or OO. At least one parent must be DD or Dd (one parent could be dd)

for A,B,O:

Punnet square for Rh: