When I started reading this book I initially thought Oskar was autistic. However as you go through the novel, you realize his odd behaviour is a product of emotional baggage and post-traumatic stress. While we could label Oskar as eccentric, I think being eccentric is more of a conscious choice to be different. For Oskar it is not a choice. Consider his behaviour:
- Obsessive compulsive behaviour: the need to find the lock that matches the key he found in his fathers room
- Avoidance: he blocks out his mother and seeks interactions with distant individuals; whether it be writing letters to Stephen Hawking or any of the secondary characters we meet throughout NYC
- Depression: Oskar's book of "things that have happen to me" is a haunting visual representation of how he feels inside.
These are all symptoms of post-traumatic stress. While Oskar's journey through the five boroughs of NYC obviously helps him process this pain, I don't think it would appropriate to merely call him "eccentric".
When it comes to Oskar's intelligence, there is no doubt he is gifted. This is exhibited right at the beginning of the novel when we learn about the interactions he had with his father before his death.