If one parent has type o and the other type a can their children have type ab positive ?

No: The child could be A or O, but since neither parent has a "B" gene, there is no way for the child to get this. "Positive" is about the rH factor, and since you have not given me the information on whether the parents are positive or negative, I can't comment on that. But if even one parent is "positive", then the child can be because that is a dominant gene.