My blood type is O-, my alleged father is AB- Is this possible? He has had a blood transfusion before. Help!

Statistically, one in every ten thousand blood type antigen genes undergo mutation between generations. This means that for an AB individual to father an O offspring, a mutation would have to occur in the development of that one sperm cell.
This, of course, ignores clerical errors (which are the most common source of blood typing errors).

Blood transfusions make no difference, but this is not the right place to ask a question that could have such a major impact on a number of lives. There is too much room for misinformation or mistakes, and there are also unusual genetic possibilities that would mean that someone could legitimately be the parent of a child when the 'rules' say they couldn't. The fact is that there's no way for anyone here to give you a definitive answer.

If you are in question as to whether or not this man is your father, the only way to tell for sure is for both of you to undergo genetic testing.

How do you know what the blood types are? If you are relying on some tests you did in high school biology, they aren't accurate.

FWIW ... I know my blood type is O, but the card that came from the bloodbank said "A". It was a mistake.

If your information is correct, the man in question is not your father... Blood transfusions do not alter an individual blood group antigen that determine blood type.

Based on your blood work info, no. However, there are many other factors and mistakes that could play in, so please see a genetic counselor or other medical professional.

I would go ask a doctor. the RH factor would work so may be. However I would ask a professional about that one.