You always hear that a girl might not know the father of her baby but how come you never hear a father wondering who the mother of his baby?
The baby's blood type could be either A or O. It depends on if the father is AA or AO. If he is AA, the baby would be type A (AO, since the baby would get an O from the mother). If the father is AO, the baby could be either AO or OO depending on if the baby inherits the A or the O from the father. (The mother will always contribute an O.)