The significance of Oskar's grandfather to the plot of this novel lies in the way in which he, like so many other characters, appears to be lost in his own grief and depression. It is his relationship with Oskar that seems to revitalise him to a certain extent when he returns after leaving before his own son is born. In a novel that is so much about second chances in one way or another, the grandfather is yet another of the multitude of bizarre characters, who chooses not to speak any more and only uses the two words "YES" and "NO" that are tatooed on his palms and a notebook to communicate.
Yet he comes to guide and help his son Oskar when he returns to his wife after an absence of so many years, and it is he who hears the final message of his son who he had never even met before he died at 9/11. Note the grandfather's response to hearing this message:
The message was cut off, you sounded so calm, you didn't sound like someone who was about to die, I wish we could have sat across a table and talked about nothing for hours, I wish we could have wasted time, I want an infinitely blank book and the rest of time.
In spite of his grief and regret, the grandfather gets a second chance with his grandson, Oskar, and has the opportunity to right the wrongs of his desertion through forming a relationship with his grandson.