Thanks Kevin, that's what I was hoping. I wanted to get a confirmation because the O-365 integration guide at couple of places says:
(Please see the text in *bold red*)
It is important to note that if another technology is performing the
synchronization of accounts to Office 365, and Okta is handling the
federation for authentication, *you need to ensure the Okta account
usernames match the Office 365 usernames*. This can easily be configured in
Okta using Universal Directory attribute expression, this is described
later in this document.
Can you please explain why the above constraint? Even in the above
scenario, Okta does have access to the immutableid (say AD-guid).
The application user ID (in this case Office 365) does not need to match the Okta username. Sometimes it is the same or similar. The statement about the Okta username does not have to match the Office 365 username. If the user is AD mastered then yes Okta will know about the immutableID.