No problem. Blood group matching plays an important role if the boy has positive group & girl has negative group. In such case, the second baby born might suffer with Erythroblastosis foetalis (A dangerous disease which can cause death of the baby). But in your case, both boy's & girl's blood group are positive (Rh positive) and hence, there won't be any problem. And again, boy's group is 'B' & girl's group is 'O'. This combination is also not associated with any future problems.