Thanks for writing to us.
is always genetically determined.
If both the parents are actually 'O-negative', all the children will be 'O-negative'.
Having said this, O negative parents can only deliver O positive child only if tests were "inaccurate".
Or if the father had weak rhesus
D gene that is actually father is positive but tested negative due to weak Rh D gene. In these cases retesting should be done to rule out weaker reaction.
Hope that answers your query. Should there be further questions, I would be happy to assist you.