Can 2 O positive parents give birth to A positive child

Here's good research: Blood Type Genetics and Compatibility - Baylor Scott & White Health (http://www.sw.org/blood-center/blood-types)

If I remember my Physiology classes, No, one would have to be an A type. Of course, I would have a DNA test done. Try this link
Blood Type Child Parental Calculator -- EndMemo (http://www.endmemo.com/medical/bloodtype.php)

There are more to blood grouping than just the O or A, There are in reality, two typings where there are a lot of combinations.

So without knowing the full typing, there is no way to answer the question.

With the very limited informaiton given, since there is a sub typing, that could have been A, it is possible.

Two O parents will produce a type O child.

BLOOD TYPE CHART_CHILD FATHER MOTHER-PATERNITY determination by blood type - parents & child (http://canadiancrc.com/paternity_determination_blood_type.aspx)