Mary O'Hare is the wife of the narrator's war buddy, Bernard V. O'Hare. She is initially furious with the narrator because she thinks this great Dresden book he's writing is going to be a celebration of war and of his own experiences as a POW. (Hahaha—no.)
She thinks that the more attractive writers and filmmakers make war seem, the more stupid wars we're going to have. The narrator promises her he will call the book The Children's Crusade. (Check out "What's Up With the Title?" for more on this.) Mary O'Hare is also one of the two characters (along with Gerhard Müller) to whom the narrator dedicates the book, presumably in recognition of her passionately anti-war feelings.
A side note: Mary O'Hare is also a nurse, which the narrator calls "a lovely thing for a woman to be" (1.10.1). In general, the narrator seems to have only good things to say about the medical profession, which he contrasts pretty obviously with the whole army apparatus he is writing against.
Probably the strongest example of this medicine-life/army-death opposition happens when Billy Pilgrim is in the hospital with a fractured skull and Bertram Copeland Rumfoord keeps telling his wife that the doctors should just pull the plug because Billy is a waste of space. The doctors refuse because they are "devoted to the idea that weak people should be helped as much as possible, that nobody should die" (9.14.3). For more on Rumfoord as an extreme example of pro-army feeling, check out his "Character Analysis."