Answer:
Email
Step-by-step explanation:
The email is the most formal way, so it is the one communication facility that Oliver would use to send a formal message to his boss.
If it was something like a quick question, the chat would be appropriated. However, a report is something bigger and more "important", more formal, so in this case the chat is not ideal.
Forum and blogs are usually not even private. So for something that is just between Oliver and his boss it would bot be appropriated.