What does Oskar learn from his quest around New York?

Oskar learns the value of friendship and honesty. He learns that sometimes you have to put effort into relationships, he learns to be independent and resourceful, and he learns to fogive himself.

Another thing I decided was that I would be as secretive about my mission as I could at home, and as honest about it as I could outside home, because that's what was necessary.

..... at the end of my search I wanted to be able to say: I don't know how I could have tried harder.

He needed me, and I couldn't pick up. I just couldn't pick up. I just couldn't. Are you there? He asked eleven times. I know, because I've counted."