One of Ours is available to read for free at Gutenberg.org
One possible meaning of the title is that it represent Claude's search for a place where he feels at home. For most of his life, he is alienated and disassociated from the people around him; he cannot find peace and joy, even in marriage, and finds himself looking forward at some higher goal and purpose than whatever his present entails. Claude does not feel connected to any of the communities he joins; he does not feel like he is "one of ours," instead being emotionally outside of the closely-knit social groups.
He felt only one thing; that he commanded wonderful men. When David came up with the supports he might find them dead, but he would find them all there. They were there to stay until they were carried out to be buried. They were mortal, but they were unconquerable.
(Cather, One of Ours, gutenberg.org)
At the end of the book, he finds camaraderie with his army platoon. Claude discovers that he is at home with them, part of their social structure and able to connect on the field of battle better than he could with his own family. Some of his struggle may come from Claude's innate desire to accomplish great things, and when sharing a mortal battle with other soldiers, each victory seems larger and more important. Claude finds peace in war, and becomes "One of Ours" to the other soldiers.