The Goal

Set All Users Contacts and Calander to Default to Reviewer Permissions

The Solution

First we need to connect to Office 365 using power shell

Import-Module MSOnline
$cred = Get-Credential
Connect-MsolService –Credential $cred

Second iterate over the Mailboxes and set permission

Get-Mailbox | ForEach-Object {  
  Set-MailboxFolderPermission ${_}:\calander -User default -AccessRights Reviewer
  Set-MailboxFolderPermission ${_}:\contacts -User default -AccessRights Reviewer
}