Can parents have a o+ blood child if?

Yes, it's possible, as long as the mother is type AO, BO, or OO. A and B are the dominate type genotypes. A Type A person can have either AA or AO. B same way. AB would be AB, and O is OO. Each parent gives 1 of the genotype pairs to the child. For Rh, positive is dominant. It's possible for a negative and positive parent to have a negative type child, but it's rare.

More information would have been useful. Normally I deal with ABO blood group first when answering this kind of question but as the only information close to complete is the Rh group let's deal with that first. If the mother is Rh-ve she is homozygous negative and can only give her progeny the allele for Rh-ve. So, the child can have one allele for Rh-ve. The father we know is Rh+ve and his genotype may be homozygous positive or heterozygous. Nonetheless if the child is Rh+ve the father has given the child the allele for Rh+ve and we must infer the child is heterozygous.

If the child's ABO blood group is O we know the child's genotype is OO. The child's father is blood group B and it is possible for him to be the father if he has the genotype BO because he can give his progeny an O allele. We do not know the mother's ABO blood group. We must therefore guess she has at least one O allele in her genotype and can, like the father, give her child an O allele.

The Rh factor (+ or -) is inherited independently of the ABO type. If one parent is Rh+, he can pass on the Rh- or Rh+ allele. A parent who is Rh- can pass only the Rh- allele. So the child can be -- or +-; which results in Rh- or Rh+.

When one parent is type B, and we don't know the type of the other parent, then the child can be A, B, AB, or O. Since the child is type O, the father could be O, A, or B (but not AB).

I've never heard of RH blood type. So what blood type is the mother? A, B, AB or O? The child's blood type can only be the mother's or the father's type. Unless the child was adopted, and maybe the parents never told the child.