If you Google “open a shared folder in outlook online” you’ll get many results telling you how to open a shared mailbox, rather than a shared folder.

Not very helpful. You can’t open another mailbox in Outlook Online unless you have Full Access permissions. If someone has shared a folder with you or if your organization doesn’t grant Full Access permissions to shared mailboxes by policy, then you can’t use the “Open another mailbox” option.

But you can still open the folder. Here’s how…

1. Log into Outlook Online.
2. Right-click on Folders in the navigation pane on the left.
3. Select “Add shared folder”.
4. Select the mailbox that contains the shared folder(s) you want to open.

There is one complication here, though, a bizarre and non-intuitive hold-over from the earliest versions of Exchange in the 1990s: You will need at least Reviewer permissions on the root of the mailbox that contains the shared folder. If you don’t, you’ll see the mailbox in your navigation pane, but it will appear to be empty, because Exchange won’t let you get a folder listing.

The sharing mailbox’s owner can add Reviewer permissions on the root of the mailbox from within Outlook or you, the Exchange administrator, can do it from within PowerShell with this cmdlet:

Add-MailboxFolderPermission -Identity MailboxAlias:\ -User jay.carper@domain.com -AccessRights Reviewer

In this cmdlet, “MailboxAlias” is the alias of the mailbox that contains the shared folder and “jay.carper@domain.com” is the email address of the user who needs to be able to access the folder.