I was watching the film "Prisoners" where at the end a little girl is given a lethal dosage of morphine. My flat mate Ed (a med student) told me that the first thing that would kill you from a morphine OD is respiratory failure. As your rate and depth of breathing reduces eventually it's not enough to keep you alive.
The medical fix is naloxone, a drug used to counter the effects of an opiate overdose. BUT the half life of naloxone is much shorter than morphine, it's broken down quicker so patients should be kept in for at least 24 hrs for observation.