Exchange online - calendar visibility

Update the "Default" entry on each calendar with the corresponding permissions. Something like this:

$calendars = Get-Mailbox -RecipientTypeDetails UserMailbox | Get-MailboxFolderStatistics -FolderScope Calendar | ? {$_.FolderType -eq "Calendar"} | select @{n="Identity"; e={$_.Identity.ToString().Replace("\",":\")}}

$calendars | % {Set-MailboxFolderPermission -Identity $_.Identity -User Default -AccessRights LimitedDetails}