Could a man with an o blood type be the father of a child with type ab blood?

Normally, that would be impossible. AB types usually have 1 A allele (gene-version) and 1 B allele, 1 from each parent. O types normally have no A or B alleles (as these are dominant to O alleles, so a person with 1 A and 1 O allele is simply A type).  

There is rare exception (much rarer than eg adoption or infidelity) known as the cis-AB gene, which can pass on AB blood type with a single allele. But that is a whole  other story.