O'Neill talks a big game throughout the film. Such a big game, in fact, we don't blame you if you're a bit sick of him by the time that final battle rolls around.
He's a close compadre of Barnes, and if it were up to him, he'd be just like him. He doesn't spend his time in the Underworld smoking pot, but rather above ground, drinking and playing poker with Barnes. When he talks, he appears to be exaggerating everything he says, especially, for example, he explains to Elias why his squad (Elias's) should go out into the jungle and not his.
Try as he might, however, O'Neill really isn't all that tough.
We start to see that he's really just an insecure, scared little boy. We first catch a glimpse of this when he observes Bunny inside one of the village huts. Rather than partake in tormenting the residents like Bunny, and even Taylor to some extent, O'Neill just wants to get out of there. The look on his face says it all. Similarly, during the final firefight, the going gets really tough, so to speak, and O'Neill, rather than fight, pulls some dead bodies over him and hides under them to avoid the onslaught.
Like Taylor, O'Neill also has a small emotional journey himself. He's not a gun-toting killer like, say, Bunny, but he's also not one of the guys who can't handle the jungle. He thinks that all the attention he's given to Barnes, all the little favors he's done for him, will somehow pay off. They don't, and O'Neill realizes that the war is a joke, and it doesn't really matter. Barnes refuses to give him a few days of rest that he asks for (even though he's never asked for anything), and after the horrible final battle Captain Harris puts him in charge of the platoon. O'Neill has had enough, and just wants to go home.