Did the war benefit O’Brien in any way? In other words, did he learn any valuable life lessons from his participation in the Vietnam War?

Okay, I'm not sure what you mean by a "real answer". The details are in the story itself but the themes are in my previous answer. Consider reading a bit and you will see themes of loyalty and isolation. The soldier's life moves from the mundane to the terrifying in a split second. What do you think he might learn? What might he learn from trampled rice paddies and burnt villages? THe terror tunnels were full of great learning moments.