Paternity and blood type???

If your mum is truely O and you dad is truely B then they shouldn't be able to produce an A child. I would get everyone rechecked. There are some very weak group A subtypes that can almost group as a group O. If your mum was grouped many years ago when the blood grouping reagents weren't as good it is possible she was misgrouped as an O when she is a weak A. The other option is that your dad is a (weak)AB. These kinds of mistakes can happen when people are doing their own blood groups from kits (and only doing forward groups and not reverse groups). With these weak subtypes, we usually find discrepancies between the forward group and reverse group.