If the mother has typeo+ and the child has b- what would the fathers be?

How are ABO alleles (http://library.thinkquest.org/20465/genes.html) inherited by our children?

Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type O can only pass an O allele to her son or daughter. A father who is blood type AB could pass either an A or a B allele to his son or daughter. This couple could have children of either blood type A (O from mother and A from father) or blood type B (O from mother and B from father).

With the mother being of type O+
And the child being B-
The biological father could possibly either be
B or AB with rh factor + or -

Hope this helps.