Offline address book not updating
You can use this switch to run tasks programmatically where prompting for administrative input is inappropriate.
You don't need to specify a value with this switch.
By default, a new installation of Exchange creates an OAB named Default Offline Address Book on the server.
This OAB is also the default OAB, which means it's the OAB that's used by mailboxes and mailbox databases that don't have an OAB assigned to them.
For information about the parameter sets in the Syntax section below, see Exchange cmdlet syntax (https://technet.microsoft.com/library/bb123552.aspx).
You need to be assigned permissions before you can run this cmdlet.
Exchange Online only updates the Default Offline Address Book (OAB) once a day, and can take another day after that to be available to users.
So unfortunately as per this Microsoft article, ‘It can take 24 to 48 hours for the GAL to update the OAB.
OAB generation is controlled by the mailbox assistant named OABGenerator Assistant that runs under the Microsoft Exchange Mailbox Assistants service.
So a typical address book update might look like this: So the process can easily take up to 24 hours before the new users are visible in the address book.
The best you can do with Office 365, is to wait until the next day after 5am, and then either restart outlook or force the OAB download using the below method.
OAB generation occurs in a designated arbitration mailbox that has the . To change the OAB generation schedule, see Change the offline address book generation schedule in Exchange Server.
To manually update an OAB, see Use the Exchange Management Shell to update offline address books.