Oppy spends most of the book on arguments for OCMOG. These he ably dismantles. (Anyone planning on debating William Lane Craig ought to memorize this book.) Why, then, does he think it is not irrational for a theist to remain theist or an agnostic to remain agnostic? The basic idea is that people start from different "priors" and have access to different evidence, so it is by no means surprising that they will come to different, and even incompatible, beliefs. And they can do so in a completely rational manner. (Not that they always do, of course.)
A rational agent will persist with the views she has until she is shown that she can improve her view by changing it.So the question for the atheist is whether he has arguments that are rationally compelling to the theist, just as the question for the theist is whether he has arguments that are rationally compelling to the atheist. And Oppy thinks the answer is "no."
Although I agree that people can differ without being irrational, and I am willing to entertain the idea that that might even be true about the existence of God, I think Oppy sets the bar for rationality too low. I wonder if he would insist that astrology believers, UFO enthusiasts, and Bigfoot hunters make no mistakes of rationality.