Days Gone - Secret Ending
What I did for this ending with O'Brien to unlock was locate the last NERO injectors after I'd completed the main story. It might not have been the injectors that triggered it but if not then you just have to do the quests given to you by Rikkie, yourself and Weaver + go do some stuff marked on the map, like killing hords.
DAYS GONE (PS4)
https://store.playstation.com/#!/en-d...