If you have blood type O neg and your wife has pos how is it possible for baby to B pos?

If the mother is Rh-positive and the father is Rh-negative, the baby can be either Rh-positive or Rh-negative. This is because the mother can pass on either her Rh-positive or Rh-negative allele to the baby. If the baby inherits the Rh-positive allele, then the baby will be Rh-positive. If the baby inherits the Rh-negative allele, then the baby will be Rh-negative.

In the case where the mother is Rh-positive and the father is Rh-negative, there is a 50% chance that the baby will be Rh-positive and a 50% chance that the baby will be Rh-negative.