What happens when b positive woman marry a o negative man?

The blood type of any children would depend on the zygosity in the woman. (Both O and Rh negative are recessive, and will be homozygous in the man).

If the woman is homozygous B, then the children will B.

If the woman is heterozygous B, either O or B (with 50% chance of each).

In the same way, if the woman is homozygous Rh positive, then the children will also be positive. If the woman is heterozygous, then children will be either positive or negative.

In short, the possibilities for child blood types are either B or O, with Rh positive or negative.