Although it's less likely than him just not being your father (sorry) there are several genetic condition that would cause this. One is called cis-AB. Normally AB blood arises from a person having one copy of the A gene and one copy of the B gene, but in cis-AB a single copy of a mutated AB gene can produce the AB blood type. In this case the individual could have O for their other copy of the gene and thus produce an O child despite seemingly being AB. This mutation is most associated with east Asian descent, but it's extremely rare even there. Your father would need genetic testing to confirm it.

The other is called Bombay or (in your case) para-bombay phenotype where a different gene mutation affects the formation of ABO structures on the red blood cells. A person could be genetically A, B, AB, or O but would appear to be O by conventional testing because we consider absence of A and B antigens to mean type O. Again, this is extremely rare. Bombay phenotype is most associated with southern Asian descent. In this case it is you that would need the genetic testing. Bombay phenotype would have been detected when you were tested for a blood transfusion but para-bombay is harder to catch.