DOA (for Dead On Arrival) is hospital "shorthand" for a patient who arrives by ambulance and has no vital signs or brain activity. In such cases, although the emergency room physician will consider resuscitation efforts, they may in appropriate circumstances, such as an emergency triage situation, take no further action to treat the patient.
As a "DOA" patient did not technically die under a physician's care, in most cases, an autopsy will be mandatory even when the cause of death is obvious.
In the episode "Help Me" Hanna a victim of the crane accident is stuck under a pile of debris and has to amputate. She DOA at the hospital.