What would children blood group be if mother A Neg and father O RH Pos?

The mother has Type A blood which means she is either AA or AO. The father has Type O which means he has OO blood type genes. So, overall the child has a 75% chance of being Type A ( either AA or AO) and a 25% chance of being type O (OO).

Similarly, the mother can only pass on a Rhesus negative gene but the father can potentially pass on either so as he might be ++ or +-. So overall, a 75% chance of being positive and a 25% chance of being negative.