Can both parent have blood type O positive have a children with typeb B?

Not if both parents are O.

There are three alleles (flavors of gene) for blood type: A, B, and O. Everyone gets two alleles, one from each parent. Depending on which allele is dominant, that determines what blood type someone is.

If you get an A from your mother and an A from your father, you are AA and thus have blood type A, i.e. the expressed blood type is A. Similarly:

A + A = AA (expressed blood type: A)

A + B = AB (expressed blood type: AB because A and B are both dominant)

A + O = AO (expressed blood type: A because A is dominant)

B + A = AB (expressed blood type: AB)

B + B = BB (expressed blood type: B)

B + O = BO (expressed blood type: B)

O + A = AO (expressed: A)

O + B = BO (expressed: B)

O + O = OO (expressed: O)

As you can see, someone whose expressed blood type is O (which is rare) has two O alleles, OO. These two people can only have children with the expressed blood type of O, as a result.

However, if one parent is BO (expressed: B) and the other parent is BO, AO, or OO, then it is possible for the children to be: AB (AB), AO (A), BO (B), or OO (O).