Ben, you can first automatically back up the items in Outlook for Mac using Time Machine (office.microsoft.com/.../automatically-archive-or-back-up-outlook-items-HA102928254.aspx), then use a network drive software like DropBox to sync the items to the net drive.
Also, O365 forum focus more on the cloud service settings part, for the Outlook clients, you can get dedicated help from MS Office community (answers.microsoft.com/.../macoffice2011-macoutlook).