If both parents have o+ blood type can their child have a+?

No. This is not possible with these blood types. I would check the parents blood types again.
If they adopt one. The genetics of blood groups is rather straight forward. The only way to pick up a a gene is to have one in the donor.I find an occasional adult who actually doesn't know their blood type or was told the wrong type in the past. The days of getting a blood transfusion based on history or even recorded on your dog tags is long gone. Too much risk of transfusion rxn. All are rechecked at time of need.