As we read this compelling story, it becomes clear that Oskar is a character who is troubled in a number of areas in his life. In particular, he finds it very difficult to understand the world around him and why it is the way it is. We can also see, however, that he, as any normal child would, is struggling to accept the death of his father and in particular to allow both himself and others around him to move on. Consider one emotional outburst he has at his mother's boyfriend, Ron:
...he took my yo-yo off my desk and started to walk the dog with it. I know he just wanted to be friendly, but it made me incedibly angry. "Yo-yo moi!" I told him, grabbing it back. What I really wanted to tell him was "You're not my dad, and you never will be."
Clearly we can see that this outburst derives from his feelings of loss and anger at having lost his father, which he is now taking out against Ron. However, Oskar also has emotional outbursts when he is unable to cope with what is going on around him and he just needs to shut down for a while.