First of all nothing to worry. Both baby and mom will not be having issues. No matter what blood group and what Rh group they belongs to. There wont be any problem for baby and mom at-least for first pregnancy. read more
In the particular case you are asking about, with both parents being O negative, this will not be a problem, at least from an ABO and Rh standpoint, because there is no way the baby can produce any antigens (i.e. Rh antigens) that are foreign to the mother. Both parents have the same blood type. read more
One thing to clarify to clarify is that we are two brothers and two sisters and all the six members of my family have same blood group O positive. My elder sister got baby after 8 years and her husband is B positive. And my younger sister, she always gets sick since her antibody of her blood cell might not strong enough to fight disease. read more