There are many levels of OD's... which is why it is often such a difficult topic to fully understand. Many people have misconceived notions about overdoses. When it comes to opiate OD's, sometimes they will be moderate enough to the point where you wont even loose consciousness.. you may just get some really nasty side effects like throwing up, sweating, etc.
Another level of OD is when one does pass out for a bit, but comes out of it all on their own. However, there is a fine line between a dose that results in that outcome compared to ones that put you out, where you will not wake up just on your own.
Even further down the later of severity when it comes to opiate overdoses are the ones where you actually need chemical help to come out of it in the form of naloxone. During an OD of this nature, you weren't coming out of it any other way and if nobody would have hit you with narcan, you would have died. Sometimes though, the OD is so severe that the individual requires multiple doses of narcan.
So you see, there is no "one size fits all" OD... they're are varying levels of severity.