Well, what’s your mom’s blood type? It’s perfectly possible for an O father to father an AB child with an AB mother. Some of their kids would be expected to be A’s, some B’s and some AB’s. You wouldn’t expect any O’s at all, though.
Also, remember that all these blood types refer to proteins found on the surface of the blood, and are caused by mutations in people’s DNA. It’s theoretically possible for a fertilized egg to develop the mutation that causes A and B antibodies spontaneously - after all, the reason we have them in our population is that, at some point, someone’s DNA spontaneously mutated to make it happen. Granted, it would be pretty unlikely for both the A and the B mutations to happen to one person spontaneously, but if your mom is an A, you could inherit that from her and be a spontaneous B mutation, making your blood AB.
Mutation is possible, but my bet would be on a dogtag mistake.
As for an O-type to be able to father an AB child without mutation, my Mendel’s Laws are rusty, but I’d be willing to place a bet against that proposition also.
I did some digging for cites in a previous thread im having trouble finding now but several reliable looking studies ran 3%-25% mainly varying with socioeconomic class. The weighted average they gave was like 7%.
Its not much of a stretch to imagine that 7% of sex is with the non obvious partner therefore 7% of the children concieved were fathered by the same non obvious partner.