O'Brien alternates between the grandiose and the conversational for effect. For example, in "The Things They Carried," we have this sentence:
They carried the sky. The whole atmosphere, they carried it, the humidity, the monsoons, the stink of fungus and decay, all of it, they carried gravity. (The Things They Carried.39)
The list begins with something pretty ambitious—"They carried the sky"—and continues from there, grandly building momentum with each added uncarry-able thing that the men must somehow carry. Finally, impossibly, at the end of the list, he tells us that the men carry gravity. It's an epic list, and it makes the men's task feel epic. Of course, in the previous section, there's also this exchange:
Henry Dobbins thought about it.
Yeah, well, he finally said. I don't see no moral.
There it is, man.
F*** off. (The Things They Carried.35-38)
The style here is brutally conversational. Grammatical niceties are not observed—"I don't see no moral"—and it does not censor conversational tics such as an unnecessary "man" at the end of a sentence or casual cursing. It feels like something you would hear on the subway.
The combination of epic, beautiful sentences that you would definitely not hear on the street and sentences that would not be out of place in a high school hallway gives the impression that O'Brien is in love with language of all types, and will use whatever style he needs to make the content ring true.