There is a clear connection between Johnathan Safan Foer's novel and Joseph Campbell's articulation of the "Hero's Journey." There are twelve steps that a character must go through on this journey to heroic status, and three distinct phases of that journey.
Part 1 begins in the "Ordinary World": For Oskar, this part of the journey is incredibly brief. His idyllic childhood with his heroic (to him) father is cut short and Oksar is thrust into the "Call to Adventure" begins, as he searches for the lock that his father's key will open.
Part 2, the "Special World" is fraught with tests as Oskar is disappointed time and again in his quest to find the lock.
In Part 3, the "Road Back" to the "Ordinary World," Oskar has discovered the lock but the discovery is unsatisfying. The "resurrection" comes when he, and his discovered-grandfather, unearth the empty coffin and fill it with mementos from Oskar's father's life. As he fills the coffin, Oskar imagines all the events in reverse, until, in his mind, his father is alive again.