There is no official Microsoft scenario where an .opk file is required to access a Hotmail account. If a tech support agent asked you to download one, they are scamming you. Hang up immediately.
Microsoft provides a free tool called SaRA (Support and Recovery Assistant). This tool automates the repair of Outlook profiles. hotmail.opk
If you are certain the file is legitimate (e.g., you are a system administrator working with legacy Windows deployment tools), here is how to handle it. There is no official Microsoft scenario where an