Involuntary warfare
The irony of O'Brien's participation in the Vietnam War was that it was perfectly involuntary. He was mandated to go to war without his permission. In fact, he opposed the war on moral grounds before his draft, and the involuntary nature of the war was proof of that moral opposition, because he knew that it was not a popular war, but yet, the government was pushing to win it. When he arrived, he was forced to make decisions about what he would and wouldn't do, but the military was paying close attention. He was forced to be a soldier.
Survival and self-preservation
Another irony arrives when O'Brien sees real conflict. His moral conundrum is simplified in a horrifying way. The question isn't what he wants to do or what he can defend to himself morally. The question is "Does he want to survive?" and the answer is yes. So, he does what it takes to survive, following orders when he has no other choice. When enemy soldiers start shooting at him to kill him, his decision isn't about US versus Vietnam; it is his life or the person trying to kill him.
Personal hell and loneliness
Loneliness emerges in ironic ways through the plot. He is physically removed from his community, so that loneliness is obvious, but there is the painful loneliness of knowing too much about the world. When he goes home, no one will be able to empathize with him, and he knows that he is a victim of the war, but he won't be treated that way. No one will know the truth and the loneliness starts to be its own hell. The ironic consequence of his suffering is PTSD and private emotional torture.
Veteran mistreatment
The veterans share a moral weight that no one understands. There are people in their communities at home who view them with contempt, not knowing that they were forced to either die or kill, and the mistreatment is even more painfully ironic because O'Brien explains how he and the other veterans are the true victims of government greed, but others treat them as the agents for that government greed. The war took innocent people and made them innocent and then people judged them.
Truth and war
When O'Brien is assigned to an investigative task, he sees an opportunity to feel hopeful. He realizes that there is still justice in the world (but not much of it). He sees that his point of view is helpful because he says the truth, and he starts to feel a calling (which the reader knows about through dramatic irony; O'Brien is a critical voice in literature about Vietnam or about warfare). He describes his emotional journey toward becoming a truth-teller. The reader knows he will end up becoming the famous author who pens this story in the first place.
