Before Oskar's dad died he left 5 messages on their phone while he was in the building. Oskar listened to the last message when he was in the room but he was scared to answer the phone. He wanted to hide the phone so his mom wouldn't know.
When Oskar found the envelope with the word "Black" on it he took it to the store to see if he could figure anything about it.
Mr. Black was one of the more interesting people Oskar met. He was a war veteran that had a note card of every significant person he thought was important.
When "The Renter" and Oskar met they were both in Oskar's grandmas apartment. Oskar didn't know who he was and he thought he was going to try to kill him. they wound up talking about the journey he had gone on and what had happened.
Who are you?
Oskar thought that he finally found out what the key was for and he thought it meant to dig up his fathers grave. he had "The Renter" help him dig it up.