Final answer:
Administering oxygen is the priority nursing action for nonreassuring or ominous signs. Oxygen helps in treating carbon monoxide poisoning by turning carboxyhemoglobin back into hemoglobin.
Step-by-step explanation:
The priority nursing action for nonreassuring or ominous signs involves administering oxygen by mask. The correct option is a) Oxygen, 5 liters. Oxygen is crucial in treating carbon monoxide poisoning as it hastens the dissociation of carbon monoxide from carboxyhemoglobin, turning it back into hemoglobin. Other options like nitrous oxide, carbon dioxide, and helium are not suitable for this purpose.