It can be possible because the mother can present B blood phenotype. This opens the possibility of she having an heterozygous genotype of I^b i or I^b I^b which are dominant over the genotype of the O type father.
Mutations are simply a change in genes. So mutations usually result in variation of an organism that can be good or bad. Some mutations can result in physical and mental disorders like Stockholm disease or down syndrome, Where else some mutations can cause immune to diseases that are harmful.