How does O negative affect pregnancy risks like hemolytic disease of fetus and newborn?

If a mother is Rh-negative (for example O-negative means ABO type O and Rh D negative) and the fetus is Rh-positive, there is a risk of Hemolytic disease of the fetus and newborn (HDFN) because the maternal immune system may develop anti-D antibodies that cross the placenta and destroy fetal red blood cells. For an O negative mother this risk is present if she carries an Rh positive fetus and has not been sensitized or given prophylaxis. 

The ABO part (type O) also means the mother’s blood group O may carry anti-A and anti-B antibodies, but these typically are IgM and do not cross placenta significantly; so the major risk is Rh D incompatibility rather than the O part alone. Women who are O negative are thus monitored for Rh incompatibility and often given Rh immunoglobulin prophylaxis to reduce risk of HDFN.