Can a o- mother and o positive father have an AB baby?

No, an O mother and an O positive father cannot have an AB baby.

A child's blood type is inherited from their parents. Each person has two alleles for blood type, one inherited from each parent. The O allele is recessive, meaning that it must be inherited from both parents in order to be expressed. The A and B alleles are dominant, meaning that if a person inherits one copy of either allele, they will have that blood type.

In order for a child to have AB blood type, they must inherit one A allele and one B allele. Since an O parent can only pass on O alleles, it is not possible for an O mother and an O positive father to have an AB baby.