Can a child have o positive blood if the mother is A positive and the father is A negative?

Yes. Regarding blood type, each person has two blood group genes--one from each parent. The blood types sort like this: For blood type O: O/O For blood type A: either A/A or A/O For blood type B: either B/B or B/O For blood type AB: A/B Thus two parents who are A/O (and are therefore blood group A) can have a child who is O/O if they both give the O gene, resulting in blood group O. Regarding Rh type, each parent also give one gene, either Rh - or Rh +. An Rh + person can be either +/+ or +/-: and Rh - person is -/-. To summarize: a mother who is A/O and Rh +/- (and thus A positive) and a father who is A/O and Rh -/- (and thus A negative) can have a child who is O/O and Rh +/- (and thus O positive).