Can a couple who both have o negative blood type have a child that has o positive blood?

Blood type. Assumming paternity is not in doubt, the rh antigen for one of the parents may be more subtly expressed on routine blood type testing. More detailed tests can be done to diisclose which parent is rh "+" and explain the apparent discrepancy.
See below. The transmission of the rhesus component (- and +) is a little more complicated than the transmission of the ABO blood type. This means that it may be possible to have a positive child from negative parents.