O'Brien switches back and forth between narrative voices, making the question about what's real and what isn't even more confusing. You start the book with "The Things They Carried," and you think it's a book told in the third person about a bunch of guys. Simple enough.
Then suddenly—bam—in "Love," it switches to a first-person central narrator; O'Brien is talking to one of the characters from "The Things They Carried," and it becomes clear that he was there the whole time during that first story, unnamed.
Mostly, the book is in the first person with the use of a peripheral narrator—there are only a few stories in which O'Brien is an active participant, as discussed in his character analysis.
But there's still one more time where O'Brien switches to the all-knowing third person narrative voice, and that's in "In the Field," when he's admitting his responsibility for the death of Kiowa. In this case, the third person increases the feelings of pain and guilt. By forcing distance in between himself and his actions in the field with Kiowa, we can see just how much it hurts him.