To be o blood group the child has to receive an o allele from each parent as o is recessive. From the info that you have given, it is obvious that the wife blood group B is not pure breeding but is a carrier for o, making her genotype Bo. Hence the o group child. Regarding the rhesus factor, which is a separate issue from the ABo alleles, a child only needs to inherit one positive allele to be pos group as Pos is dominant to recessive. This child has obviously inherited his/her dominant allele from mum as dad is pure breeding for the recessive neg. Allele.